Comparing amyloid-β deposition, neuroinflammation, glucose metabolism, and mitochondrial complex I activity in brain: a PET study in aged monkeys

The development status of PET radiotracers for evaluating neuroinflammation

Neuroinflammation is associated with the pathophysiologies of neurodegenerative and psychiatric disorders. Evaluating neuroinflammation using positron emission tomography (PET) plays an important role in the early diagnosis and determination of proper treatment of brain diseases. To quantify neuroinflammatory responses in vivo, many PET tracers have been developed using translocator proteins, imidazole-2 binding site, cyclooxygenase, monoamine oxidase-B, adenosine, cannabinoid, purinergic P2X7, and CSF-1 receptors as biomarkers. In this review, we introduce the latest developments in PET tracers that can image neuroinflammation, focusing on clinical trials, and further consider their current implications.

Mitochondrial complex I density is associated with IQ and cognition in cognitively healthy adults: an in vivo [18F]BCPP-EF PET study

BACKGROUND

Mitochondrial function plays a key role in regulating neurotransmission and may contribute to general intelligence. Mitochondrial complex I (MC-I) is the largest enzyme of the respiratory chain. Recently, it has become possible to measure MC-I distribution in vivo, using a novel positron emission tomography tracer [18F]BCPP-EF, thus, we set out to investigate the association between MC-I distribution and measures of cognitive function in the living healthy brain.

RESULTS

Analyses were performed in a voxel-wise manner and identified significant associations between [18F]BCPP-EF DVRCS-1 in the precentral gyrus and parietal lobes and WAIS-IV predicted IQ, WAIS-IV arithmetic and WAIS-IV symbol-digit substitution scores (voxel-wise Pearson's correlation coefficients transformed to Z-scores, thresholded at Z = 2.3 family-wise cluster correction at p < 0.05, n = 16). Arithmetic scores were associated with middle frontal and post-central gyri tracer uptake, symbol-digit substitution scores were associated with precentral gyrus tracer uptake. RAVLT recognition scores were associated with [18F]BCPP-EF DVRCS-1 in the middle frontal gyrus, post-central gyrus, occipital and parietal regions (n = 20).

CONCLUSIONS

Taken together, our findings support the theory that mitochondrial function may contribute to general intelligence and indicate that interindividual differences in MC-I should be a key consideration for research into mitochondrial dysfunction in conditions with cognitive impairment.

Cerenkov luminescence imaging of interscapular brown adipose tissue using a TSPO-targeting PET probe in the UCP1 ThermoMouse

Rationale: [¹⁸F]fluorodeoxyglucose-positron emission tomography ([¹⁸F]FDG-PET) has been widely used as an imaging technique to measure interscapular brown adipose tissue (iBAT) activity. However, it is challenging to obtain iBAT-specific images using [¹⁸F]FDG-PET because increased uptake of [¹⁸F]FDG is observed in tumors, muscle, and inflamed tissues. Uncoupling protein 1 (UCP1) in the mitochondrial membrane, a well-known molecular marker of BAT, has been proposed as a useful BAT imaging marker. Recently, the UCP1 ThermoMouse was developed as a reporter mouse for monitoring UCP1 expression and investigating BAT activation. In addition, Translocator protein-18 kDa (TSPO) located in the outer mitochondrial membrane is also overexpressed in BAT, suggesting that TSPO-targeting PET has potential for iBAT imaging. However, there are no studies monitoring BAT using TSPO-targeting PET probes in the UCP1 ThermoMouse. Moreover, the non-invasive Cerenkov luminescence imaging (CLI) using Cerenkov radiation from the PET probe has been proposed as an alternative option for PET as it is less expensive and user-friendly. Therefore, we selected [¹⁸F]fm-PBR28-d₂ as a TSPO-targeting PET probe for iBAT imaging to evaluate the usefulness of CLI in the UCP1 ThermoMouse.

Methods: UCP1 ThermoMouse was used to monitor UCP1 expression. Western blotting and immunohistochemistry were performed to measure the level of protein expression. [¹⁸F]fm-PBR28-d₂ and [¹⁸F]FDG were used as radioactive probes for iBAT imaging. PET images were acquired with SimPET, and optical images were acquired with IVIS 100.

Results: UCP1 ThermoMouse showed that UCP1 and TSPO expressions were correlated in iBAT. In both PET and CLI, the TSPO-targeting probe [¹⁸F]fm-PBR28-d₂ was superior to [¹⁸F]FDG for acquiring iBAT images. The high molar activity of the probe was essential for CLI and PET imaging. We tested the feasibility of TSPO-targeting probe under cold exposure by imaging with TSPO-PET/CLI. Both signals of iBAT were clearly increased after cold stimulation. Under prolonged isoflurane anesthesia, TSPO-targeting images showed higher signals from iBAT in the short-term than in long-term groups.

Conclusion: We demonstrated that TSPO-PET/CLI reflected UCP1 expression in iBAT imaging better than [¹⁸F]FDG-PET/CLI under the various conditions. Considering convenience and cost, TSPO-CLI could be used as an alternative TSPO-PET technique for iBAT imaging.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [¹⁸F]-FDG and NA [¹⁸F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of ¹¹C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

Mitochondrial complex-I abnormalities underlie neurodegeneration and cognitive decline in Alzheimer's disease

BACKGROUND

Abnormal mitochondrial metabolism has been described in Alzheimer's disease (AD) brain. However, the relationship between AD pathophysiology and key mitochondrial processes remains elusive. The purpose of this study is to investigate whether mitochondrial complex I dysfunction is associated with amyloid aggregation, or glucose metabolism and brain atrophy in patients with mild AD using positron emission tomography (PET).

METHODS

Amyloid and tau positive symptomatic AD patients with clinical dementia rating 0.5 or 1 (N=30; mean age ± standard deviation: 71.8 ± 7.6 years) underwent magnetic resonance imaging and PET scans with [¹⁸ F]BCPP-EF, [¹¹ C]PiB and [¹⁸ F]FDG for assessing brain atrophy, mitochondrial complex I dysfunction, amyloid deposition, and glucose metabolism, respectively. Local cortical associations among these biomarkers and gray matter volume were evaluated with voxel-based regressions models.

RESULTS

[¹⁸ F]BCPP-EF standardized uptake value ratio (SUVR) was positively correlated with [¹⁸ F]FDG SUVR in the widespread brain area, while its associations with gray matter volume were restricted to the parahippocampal gyrus. Reductions in [¹⁸ F]BCPP-EF SUVR were associated with domain-specific cognitive performance. We did not observe regional associations between mitochondrial dysfunction and amyloid burden.

CONCLUSIONS

In symptomatic cases, although mitochondrial complex I reduction is linked to a wide range of downstream neurodegenerative processes such as hypometabolism, atrophy, and cognitive decline, the link to amyloid was not observable. The data presented here support [¹⁸ F]BCPP-EF as an excellent imaging tool to investigate mitochondrial dysfunction in AD.

Novel Positron Emission Tomography Radiotracers for Imaging Mitochondrial Complex I

Mitochondrial dysfunction has been indicated in neurodegenerative and other disorders. The mitochondrial complex I (MC-I) of the electron transport chain (ETC) on the inner membrane is the electron entry point of the ETC and is essential for the production of reactive oxygen species. Based on a recently identified β-keto-amide type MC-I modulator from our laboratory, an ¹⁸F-labeled positron emission tomography (PET) tracer, ¹⁸F-2, was prepared. PET/CT imaging studies demonstrated that ¹⁸F-2 exhibited rapid brain uptake without significant wash out during the 60 min scanning time. In addition, the binding of ¹⁸F-2 was higher in the regions of the brain stem, cerebellum, and midbrain. The uptake of ¹⁸F-2 can be significantly blocked by its parent compound. Collectively, the results strongly suggest successful development of MC-I PET tracers from this chemical scaffold that can be used in future mitochondrial dysfunction studies of the central nervous system.

Positron Emission Tomography in Animal Models of Alzheimer's Disease Amyloidosis: Translational Implications

Animal models of Alzheimer's disease amyloidosis that recapitulate cerebral amyloid-beta pathology have been widely used in preclinical research and have greatly enabled the mechanistic understanding of Alzheimer's disease and the development of therapeutics. Comprehensive deep phenotyping of the pathophysiological and biochemical features in these animal models is essential. Recent advances in positron emission tomography have allowed the non-invasive visualization of the alterations in the brain of animal models and in patients with Alzheimer's disease. These tools have facilitated our understanding of disease mechanisms and provided longitudinal monitoring of treatment effects in animal models of Alzheimer's disease amyloidosis. In this review, we focus on recent positron emission tomography studies of cerebral amyloid-beta accumulation, hypoglucose metabolism, synaptic and neurotransmitter receptor deficits (cholinergic and glutamatergic system), blood-brain barrier impairment, and neuroinflammation (microgliosis and astrocytosis) in animal models of Alzheimer's disease amyloidosis. We further propose the emerging targets and tracers for reflecting the pathophysiological changes and discuss outstanding challenges in disease animal models and future outlook in the on-chip characterization of imaging biomarkers towards clinical translation.

Evaluation of intracellular processes in quinolinic acid-induced brain damage by imaging reactive oxygen species generation and mitochondrial complex I activity

PURPOSE

Our study aimed to elucidate the intracellular processes associated with quinolinic acid (QA)-induced brain injury by acquiring semiquantitative fluorescent images of reactive oxygen species (ROS) generation and positron emission tomography (PET) images of mitochondrial complex I (MC-I) activity.

METHODS

Ex vivo fluorescent imaging with dihydroethidium (DHE) and PET scans with 18F-BCPP-EF were conducted at 3 h and 24 h after QA injection into the rat striatum. Immunohistochemical studies were performed 24 h after QA injection into the rat brain using monoclonal antibodies against neuronal nuclei (NeuN) and CD11b.

RESULTS

A strong DHE-derived fluorescent signal was detected in a focal area within the QA-injected striatum 3 h after QA injection, and increased fluorescent signal spread throughout the striatum and parts of the cerebral cortex after 24 h. By contrast, 18F-BCPP-EF uptake in the QA-injected rat brain was unchanged after 3 h and markedly decreased after 24 h, not only in the striatum but also in the cerebral hemisphere. The fluorescent signal in the striatum 24 h after QA injection colocalised with microglial marker expression.

CONCLUSIONS

We successfully obtained functional images of focal ROS generation during the early period of excitotoxic injury, and microglial ROS generation and mitochondrial dysfunction were observed during the progression of the inflammatory response. Both ex vivo DHE imaging and in vivo 18F-BCPP-EF-PET were sufficiently sensitive to detect the respective processes of QA-induced brain damage. Our study contributes to the functional imaging of multiple events during the pathological process.

PET Imaging of Neuroinflammation in Alzheimer's Disease

Neuroinflammation play an important role in Alzheimer's disease pathogenesis. Advances in molecular imaging using positron emission tomography have provided insights into the time course of neuroinflammation and its relation with Alzheimer's disease central pathologies in patients and in animal disease models. Recent single-cell sequencing and transcriptomics indicate dynamic disease-associated microglia and astrocyte profiles in Alzheimer's disease. Mitochondrial 18-kDa translocator protein is the most widely investigated target for neuroinflammation imaging. New generation of translocator protein tracers with improved performance have been developed and evaluated along with tau and amyloid imaging for assessing the disease progression in Alzheimer's disease continuum. Given that translocator protein is not exclusively expressed in glia, alternative targets are under rapid development, such as monoamine oxidase B, matrix metalloproteinases, colony-stimulating factor 1 receptor, imidazoline-2 binding sites, cyclooxygenase, cannabinoid-2 receptor, purinergic P2X7 receptor, P2Y12 receptor, the fractalkine receptor, triggering receptor expressed on myeloid cells 2, and receptor for advanced glycation end products. Promising targets should demonstrate a higher specificity for cellular locations with exclusive expression in microglia or astrocyte and activation status (pro- or anti-inflammatory) with highly specific ligand to enable in vivo brain imaging. In this review, we summarised recent advances in the development of neuroinflammation imaging tracers and provided an outlook for promising targets in the future.

Is Cerebral Amyloid-β Deposition Related to Post-stroke Cognitive Impairment?

Approximately two-thirds of ischemic stroke patients suffer from different levels of post-stroke cognitive impairment (PSCI), but the underlying mechanisms of PSCI remain unclear. Cerebral amyloid-β (Aβ) deposition, a pathological hallmark of Alzheimer's disease, has been discovered in the brains of stroke patients in some autopsy studies. However, less is known about the role of Aβ pathology in the development of PSCI. It is hypothesized that cerebral ischemic injury may lead to neurotoxic Aβ accumulation in the brain, which further induces secondary neurodegeneration and progressive cognitive decline after stroke onset. In this review, we summarized available evidence from pre-clinical and clinical studies relevant to the aforementioned hypothesis. We found inconsistency in the results obtained from studies in rodents, nonhuman primates, and stroke patients. Moreover, the causal relationship between post-stroke cerebral Aβ deposition and PSCI has been uncertain and controversial. Taken together, evidence supporting the hypothesis that brain ischemia induces cerebral Aβ deposition has been insufficient so far. And, there is still no consensus regarding the contribution of cerebral amyloid pathology to PSCI. Other non-amyloid neurodegenerative mechanisms might be involved and remain to be fully elucidated.

Mitochondrial complex I abnormalities is associated with tau and clinical symptoms in mild Alzheimer's disease

Background

Mitochondrial electron transport chain abnormalities have been reported in postmortem pathological specimens of Alzheimer’s disease (AD). However, it remains unclear how amyloid and tau are associated with mitochondrial dysfunction in vivo. The purpose of this study is to assess the local relationships between mitochondrial dysfunction and AD pathophysiology in mild AD using the novel mitochondrial complex I PET imaging agent [¹⁸F]BCPP-EF.

Methods

Thirty-two amyloid and tau positive mild stage AD dementia patients (mean age ± SD: 71.1 ± 8.3 years) underwent a series of PET measurements with [¹⁸F]BCPP-EF mitochondrial function, [¹¹C]PBB3 for tau deposition, and [¹¹C] PiB for amyloid deposition. Age-matched normal control subjects were also recruited. Inter and intrasubject comparisons of levels of mitochondrial complex I activity, amyloid and tau deposition were performed.

Results

The [¹⁸F]BCPP-EF uptake was significantly lower in the medial temporal area, highlighting the importance of the mitochondrial involvement in AD pathology. [¹¹C]PBB3 uptake was greater in the temporo-parietal regions in AD. Region of interest analysis in the Braak stage I-II region showed significant negative correlation between [¹⁸F]BCPP-EF SUVR and [¹¹C]PBB3 BP_ND (R = 0.2679, p = 0.04), but not [¹¹C] PiB SUVR.

Conclusions

Our results indicated that mitochondrial complex I is closely associated with tau load evaluated by [¹¹C]PBB3, which might suffer in the presence of its off-target binding. The absence of association between mitochondrial complex I dysfunction with amyloid load suggests that mitochondrial dysfunction in the trans-entorhinal and entorhinal region is a reflection of neuronal injury occurring in the brain of mild AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00448-1.

Imaging mitochondrial complex I activation during a vibrotactile stimulation: A PET study using [18F]BCPP-EF in the conscious monkey brain

In order to evaluate the capability of 2-tert-butyl-4-chloro-5-{6-[2-(2-[¹⁸F]fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF), a novel positron emission tomography (PET) probe for mitochondrial complex I (MC-I) activity, to assess neuronal activation, an activation PET study was conducted in the conscious monkey brain with a continuous unilateral vibrotactile stimulation. PET scans with [¹⁵O]H₂O, [¹⁸F]BCPP-EF, or 2-deoxy-2-[¹⁸F]fluoroglucose ([¹⁸F]FDG) were conducted under: (1) resting conditions; (2) a continuous vibration stimulation; (3) a continuous vibration stimulation after 15-min pre-vibration; and (4) a continuous vibration stimulation after 30-min pre-vibration. The contralateral/ipsilateral ratio (CIR) in the somatosensory cortex showed significant increases in the uptake of [¹⁵O]H₂O, [¹⁸F]BCPP-EF, and [¹⁸F]FDG with the vibration stimulation. The longer pre-vibration duration induced significantly lower CIR in regional cerebral blood flow (rCBF) measured using [¹⁵O]H₂O, whereas it did not affect the CIR in [¹⁸F]BCPP-EF or the regional cerebral metabolic rate of glucose (rCMRglc) measured using [¹⁸F]FDG 30-60 min after the injection. These results suggest that the [¹⁸F]BCPP-EF response in the later phase of scans was not influenced by the increase in rCBF, indicating the capability of [¹⁸F]BCPP-EF to detect acute changes in MC-I activity induced by neuronal activation. However, the metabolic shift from glycolysis to oxidation was not observed under the stimulation used here.

In vivo positron emission tomography imaging of mitochondrial abnormalities in a mouse model of tauopathy

Damaged mitochondria may be one of the earliest manifestations of Alzheimer's disease. Because oxidative phosphorylation is a primary source of neuronal energy, unlike glycolysis-dependent energy production in inflamed glia, mitochondrial respiration could provide a selective biomarker of neuronal deterioration in Alzheimer's disease. Here we used a recently developed positron emission tomography (PET) probe targeting mitochondrial complex I (MC-I), ¹⁸F-BCPP-EF, to non-invasively visualize mitochondrial abnormalities in the brains of tau transgenic mice (rTg4510). Tauopathy and neuroinflammation were visualized by PET using a tau probe ¹¹C-PBB3 and a translocator protein probe, ¹⁸F-FEBMP, respectively. A marked reduction in ¹⁸F-BCPP-EF uptake was observed in hippocampal and forebrain regions of tau transgenic mice, colocalizing with regions of tauopathy, neuronal damage, and neuroinflammation. MC-I signals were highly correlated with atrophy assayed by magnetic resonance imaging, but negatively associated with inflammatory signals, indicating that neuronal metabolic signals measured by MC-I PET were robust to inflammatory interference. MC-I may be a useful imaging biomarker to detect neuronal damage and metabolic changes with minimal interference from concomitant glial hypermetabolism.

In vivo mitochondrial and glycolytic impairments in patients with Alzheimer disease

Objective

In vivo glycolysis-related glucose metabolism and electron transport chain-related mitochondrial activity may be different regionally in the brains of patients with Alzheimer disease (AD). To test this hypothesis regarding AD pathophysiology, we measured the availability of mitochondrial complex-I (MC-I) with the novel PET probe [18F]2-tert- butyl-4-chloro-5–2H- pyridazin-3-one ([18F]BCPP-EF), which binds to MC-I, and compared [18F]BCPP-EF uptake with 18F-fluorodeoxyglucose ([18F]FDG) uptake in the living AD brain.

Methods

First, the total distribution volume (VT) of [18F]BCPP-EF from 10 normal controls (NCs) was quantified using arterial blood samples and then tested to observe whether VT could substitute for the standard uptake value relative to the global count (SUVRg). Eighteen NCs and 14 different NCs underwent PET with [18F]BCPP-EF or [18F]FDG, respectively. Second, 32 patients with AD were scanned semiquantitatively with double PET tracers. Interparticipant and intraparticipant comparisons of the levels of MC-I activity ([18F]BCPP-EF) and glucose metabolism ([18F]FDG) were performed.

Results

The [18F]BCPP-EF VT was positively correlated with the [18F]BCPP-EF SUVRg, indicating that the use of the SUVRg was sufficient for semiquantitative evaluation. The [18F]BCPP-EF SUVRg, but not the [18F]FDG SUVRg, was significantly lower in the parahippocampus in patients with AD, highlighting the prominence of oxidative metabolic failure in the medial temporal cortex. Robust positive correlations between the [18F]BCPP-EF SUVRg and [18F]FDG SUVRg were observed in several brain regions, except the parahippocampus, in early-stage AD.

Conclusions

Mitochondrial dysfunction in the parahippocampus was shown in early-stage AD. Mitochondria-related energy failure may precede glycolysis-related hypometabolism in regions with pathologically confirmed early neurodegeneration in AD.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Sigma-1 Receptor Agonists Induce Oxidative Stress in Mitochondria and Enhance Complex I Activity in Physiological Condition but Protect Against Pathological Oxidative Stress

The sigma1 receptor (σ1R) is a chaperone protein residing at mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), where it modulates Ca2+ exchange between the ER and mitochondria by interacting with inositol-1,4,5 trisphosphate receptors (IP3Rs). The σ1R is highly expressed in the central nervous system and its activation stimulates neuromodulation and neuroprotection, for instance in Alzheimer's disease (AD) models in vitro and in vivo. σ1R effects on mitochondria pathophysiology and the downstream signaling are still not fully understood. We here evaluated the impacts of σ1R ligands in mouse mitochondria preparations on reactive oxygen species (ROS) production, mitochondrial respiration, and complex activities, in physiological condition and after direct application of amyloid Aβ1-42 peptide. σ1R agonists (2-(4-morpholinethyl)-1-phenylcyclohexanecarboxylate hydrochloride (PRE-084), tetrahydro-N,N-dimethyl-5,5-diphenyl-3-furanmethanamine (ANAVEX1-41, AN1-41), (S)-1-(2,8-dimethyl-1-thia-3,8-diazaspiro[4.5]dec-3-yl)-3-(1H-indol-3-yl)propan-1-one (ANAVEX3-71, AN3-71), dehydroepiandrosterone-3 sulfate (DHEA), donepezil) increased mitochondrial ROS in a σ1R antagonist-sensitive manner but decreased Aβ1-42-induced increase in ROS. σ1R ligands (agonists or antagonists) did not impact respiration but attenuated Aβ1-42-induced alteration. σ1R agonists (PRE-084, AN1-41, tetrahydro-N,N-dimethyl-2,2-diphenyl-3-furanmethanamine hydrochloride (ANAVEX2-73, AN2-73), AN3-71) increased complex I activity, in a Ca2+-dependent and σ1R antagonist-sensitive manner. σ1R ligands failed to affect complex II, III, and IV activities. The increase in complex I activity explain the σ1R-induced increase in ROS since ligands failed to affect other sources of ROS accumulation in mitochondria and homogenates, namely NADPH oxidase (NOX) and superoxide dismutase (SOD) activities. Furthermore, Aβ1-42 significantly decreased the activity of complexes I and IV and σ1R agonists attenuated the Aβ1-42-induced complex I and IV dysfunctions. σ1R activity in mitochondria therefore results in a Ying-Yang effect, by triggering moderate ROS increase acting as a physiological signal and promoting a marked anti-oxidant effect in pathological (Aβ) conditions.

Mitochondria and aging: A role for the mitochondrial transition pore?

The cellular mechanisms responsible for aging are poorly understood. Aging is considered as a degenerative process induced by the accumulation of cellular lesions leading progressively to organ dysfunction and death. The free radical theory of aging has long been considered the most relevant to explain the mechanisms of aging. As the mitochondrion is an important source of reactive oxygen species (ROS), this organelle is regarded as a key intracellular player in this process and a large amount of data supports the role of mitochondrial ROS production during aging. Thus, mitochondrial ROS, oxidative damage, aging, and aging-dependent diseases are strongly connected. However, other features of mitochondrial physiology and dysfunction have been recently implicated in the development of the aging process. Here, we examine the potential role of the mitochondrial permeability transition pore (mPTP) in normal aging and in aging-associated diseases.

Sensitive and early detection of mitochondrial dysfunction in the liver of NASH model mice by PET imaging with 18F-BCPP-BF

BACKGROUND

Nonalcoholic fatty liver disease is a common disorder that progresses from simple fatty liver (steatosis) to nonalcoholic steatohepatitis (NASH). It is thought that mitochondrial dysfunction plays a critical role in the progression of NASH. In this study, we developed a non-invasive method for early diagnosis and staging of NASH that directly measures mitochondrial complex-I (MC-I) activity in the liver of NASH model mice by positron emission tomography (PET) imaging using the novel tracer 2-tert-butyl-4-chloro-5-[6-(4-[¹⁸F]fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one (¹⁸F-BCPP-BF). Liver uptake of ¹⁸F-BCPP-BF in NASH and age-matched control mice was measured as a standard uptake value over a period of 1 to 12 weeks. Histopathological evaluation of the liver tissue was performed by haematoxylin and eosin staining, and fibrosis was assessed by Masson's trichrome staining.

RESULTS

Significant mitochondrial dysfunction was detected as early as 1 week after commencing the diet, and MC-I activity in the liver measured by PET was reduced by > 50% relative to that in age-matched control mice after 6 weeks. Liver uptake of ¹⁸F-BCPP-BF was low throughout the 12-week experimental period. Histopathological examination revealed that steatosis, inflammation, and ballooning progressed from 1 to 6 weeks, with fibrosis observed from 6 to 12 weeks.

CONCLUSIONS

PET scans and histopathological analysis revealed that mitochondrial dysfunction in the liver contributed to the progression of NASH. PET imaging with ¹⁸F-BCPP-BF is a useful tool for detecting NASH at early stages and for monitoring therapeutic response.

Effect of MPTP on Serotonergic Neuronal Systems and Mitochondrial Complex I Activity in the Living Brain: A PET Study on Conscious Rhesus Monkeys

The objective of the present PET study was to compare the effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on serotonergic neuronal systems and mitochondrial complex I (MC-I) activity with that of dopamine in conscious rhesus monkeys (Macaca mulatta). Methods: A Parkinson disease monkey model was prepared by repeated administration of MPTP. For the PET measurements, normal and MPTP-treated conscious monkeys received an intravenous injection of ¹¹C-DASB for serotonin transporter, ¹⁸F-MPPF for serotonin 1A receptor, ¹¹C-PE2I for dopamine transporter, ¹¹C-6MemTyr for dopamine synthesis, ¹¹C-raclopride for dopamine D₂ receptor, or ¹⁸F-BCPP-EF for MC-I. Serotonin and dopamine parameters were calculated using time-activity curves in the cerebellum as the input function. The total distribution volume of ¹⁸F-BCPP-EF was assessed using Logan plot graphical analysis with metabolite-corrected plasma as the input function. Results: MPTP-induced diffuse reductions in MC-I activity were observed throughout the brain, except the cerebellum. Significant reductions in the presynaptic dopamine parameters-dopamine transporter and dopamine synthesis-were detected in the striatum and substantia nigra pars compacta of MPTP-treated monkeys, whereas no significant differences in postsynaptic dopamine D₂ receptor binding were observed. Serotonin transporter binding was reduced by MPTP not only in striatal regions but also in extrastriatal regions. In contrast, serotonin 1A receptor binding was unaffected by MPTP anywhere in the brain. In the cortex, the reduction of serotonin transporter binding correlated with that of MC-I. Conclusion: The results obtained by multiparametric PET measurements in a Parkinson disease monkey model demonstrated that chronic MPTP treatment induced reductions not only in the dopaminergic system in the nigrostriatal pathway but also in serotonin transporter in the cortical and subcortical regions. These results suggest that the neurotoxicity of MPTP is not exclusive to the nigrostriatal pathway, as predicted from MC-I damage in the extrastriatal regions of the brain.

Monitoring Mitochondrial Complex-I Activity Using Novel PET Probe 18F-BCPP-EF Allows Early Detection of Radiotherapy Effect in Murine Squamous Cell Carcinoma

Objectives

Aerobic glycolysis, the main pathway of energy production in tumors (Warburg effect) allows detection of tumors by positron emission tomography (PET) using ¹⁸F-fluoro-2-deoxy-D-glucose (¹⁸F-FDG). Since ionizing radiation (IR) is reported to switch aerobic glycolysis to mitochondrial oxidative phosphorylation, radiotherapeutic efficacy was monitored by the activity of mitochondrial complex I (MC-I), using a new PET probe ¹⁸F-BCPP-EF, ¹⁸F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoro-ethoxy)-ethoxy] -pyridine-3-ylmethoxy}-2H-pyridazin-3-one, compared with ¹⁸F-FDG uptake and the apoptosis index.

Methods

Tumor uptake of ¹⁸F-BCPP-EF or ¹⁸F-FDG was examined in C3H/HeN mice inoculated with murine squamous cell carcinoma SCCVII at various time points after a single dose of x-ray irradiation at 0, 6, 15, or 30 Gy. Apoptosis incidence was determined by TUNEL staining in excised tumor tissue.

Results

Tumor growth suppression was dose-dependent; tumor grew 10-fold (0 Gy), 5-fold (6 Gy), 2-fold (15 Gy), and reduced to half in its volume (30 Gy) 14 days after treatment. ¹⁸F-BCPP-EF uptake was significantly increased as early as 3 days after 15 Gy or 30 Gy, when tumor size and apoptosis index showed no difference among radiation doses. In contrast, ¹⁸F-FDG uptake was initially increased dose-dependently, remained elevated up to 7 days, and eventually decreased 10 days after 30 Gy and also 14 days after 15 Gy when tumor size was already reduced. Apoptosis index was increased after irradiation but failed to correlate with tumor response.

Conclusion

Tumor uptake of ¹⁸F-BCPP-EF was increased dose-dependently early after effective doses of IR when ¹⁸F-FDG uptake as well as apoptosis incidence were not indicative of tumor response. The results suggest that ¹⁸F-BCPP-EF is a promising “positive” MC-I imaging PET probe for early detection of efficacy of tumor radiotherapy.

Effects of acetaminophen on mitochondrial complex I activity in the rat liver and kidney: a PET study with 18F-BCPP-BF

BACKGROUND

In the present study, 2-tert-butyl-4-chloro-5-[6-(4-¹⁸F-fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one (¹⁸F-BCPP-BF), a PET probe for mitochondrial complex I (MC-I), was used to validate whether MC-I is a useful biomarker for detecting acetaminophen-induced dysfunctions in the liver and kidney. The kinetic and distribution of ¹⁸F-BCPP-BF were assessed in rats using high-resolution animal PET in vivo. The binding specificity of ¹⁸F-BCPP-BF to MC-I in the liver and kidney was confirmed by the pre-administration of rotenone, a specific MC-I inhibitor. The effects of acetaminophen on MC-I activity were assessed 2 and 24 h after the administration of vehicle or acetaminophen at a dose of 100 or 300 mg/kg. Biochemical parameters in plasma and urine were assessed 2, 6, and 24 h after the administration of vehicle or acetaminophen.

RESULTS

The uptake of ¹⁸F-BCPP-BF by the liver and kidney was significantly inhibited by the pre-administration of rotenone. Two and more hours after the administration of acetaminophen, the uptake of ¹⁸F-BCPP-BF was dose-dependently reduced in the liver, even at 100 mg/kg, and in the kidney at 300 mg/kg, whereas biological parameters started to be affected 6 h or later at doses of 300 mg/kg.

CONCLUSIONS

The present study demonstrated that ¹⁸F-BCPP-BF has potential as a PET probe for the quantitative imaging of hepatic and renal dysfunction as impaired MC-I activity in the early phase of the treatment for an overdose of acetaminophen in the living body with PET.

Non-invasive evaluation of neuroprotective drug candidates for cerebral infarction by PET imaging of mitochondrial complex-I activity

The development of a diagnostic technology that can accurately determine the pathological progression of ischemic stroke and evaluate the therapeutic effects of cerebroprotective agents has been desired. We previously developed a novel PET probe, 2-tert-butyl-4-chloro-5-{6-[2-(2-¹⁸F-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ([¹⁸F]BCPP-EF) for detecting activity of mitochondrial complex I (MC-I). This probe was shown to visualize neuronal damage in the living brain of rodent and primate models of neurodegenerative diseases. In the present study, [¹⁸F]BCPP-EF was applied to evaluate the therapeutic effects of a neuroprotectant, liposomal FK506 (FK506-liposomes), on cerebral ischemia/reperfusion (I/R) injury in transient middle cerebral artery occlusion rats. The PET imaging using [¹⁸F]BCPP-EF showed a prominent reduction in the MC-I activity in the ischemic brain hemisphere. Treatment with FK506-liposomes remarkably increased the uptake of [¹⁸F]BCPP-EF in the ischemic side corresponding to the improvement of blood flow disorders and motor function deficits throughout the 7 days after I/R. Additionally, the PET scan could diagnose the extent of the brain damage accurately and showed the neuroprotective effect of FK506-liposomes at Day 7, at which 2, 3, 5-triphenyltetrazolium chloride staining couldn’t visualize them. Our study demonstrated that the PET technology using [¹⁸F]BCPP-EF has a potent capacity to evaluate the therapeutic effect of drug candidates in living brain.

Contributions of Nonhuman Primates to Research on Aging

Aging is the biological process of declining physiologic function associated with increasing mortality rate during advancing age. Humans and higher nonhuman primates exhibit unusually longer average life spans as compared with mammals of similar body mass. Furthermore, the population of humans worldwide is growing older as a result of improvements in public health, social services, and health care systems. Comparative studies among a wide range of organisms that include nonhuman primates contribute greatly to our understanding about the basic mechanisms of aging. Based on their genetic and physiologic relatedness to humans, nonhuman primates are especially important for better understanding processes of aging unique to primates, as well as for testing intervention strategies to improve healthy aging and to treat diseases and disabilities in older people. Rhesus and cynomolgus macaques are the predominant monkeys used in studies on aging, but research with lower nonhuman primate species is increasing. One of the priority topics of research about aging in nonhuman primates involves neurologic changes associated with cognitive decline and neurodegenerative diseases. Additional areas of research include osteoporosis, reproductive decline, caloric restriction, and their mimetics, as well as immune senescence and chronic inflammation that affect vaccine efficacy and resistance to infections and cancer. The purpose of this review is to highlight the findings from nonhuman primate research that contribute to our understanding about aging and health span in humans.

PET Imaging of Mitochondrial Complex I with 18F-BCPP-EF in the Brains of MPTP-Treated Monkeys

(18)F-BCPP-EF was applied to assess mitochondrial complex I (MC-I) activity in the brains of Parkinson disease model monkeys prepared by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and also presynaptic dopamine parameters.

METHODS

(11)C-β-CFT for the dopamine transporter; (11)C-3,4-dihydroxy-phenyl-L-alanine (β-(11)C-L-DOPA), L-6-(18)F-fluorodopa ((18)F-FDOPA), or 6-(11)C-methyl-m-tyrosine ((11)C-6MemTyr) for dopamine synthesis; or 2-tert-butyl-4-chrolo-5-{6-[2-(2-(18)F-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF) for MC-I was intravenously injected into normal and MPTP monkeys in order to analyze their uptake in the striatum.

RESULTS

Significant reductions in presynaptic dopamine parameters and MC-I activity were detected in the striatum of MPTP monkeys. Correlations were observed between MC-I activity and dopamine transporter as well as between MC-I activity and dopamine synthesis in the striatum. The order of detectability of impaired MC-I activity was (11)C-6MemTyr >> β-(11)C-L-DOPA > (18)F-FDOPA.

CONCLUSION

(18)F-BCPP-EF has potential as a PET probe for the quantitative imaging of MC-I damage in the living brains of Parkinson disease model monkeys using PET.

In vivo molecular neuroimaging of glucose utilization and its association with fibrillar amyloid-β load in aged APPPS1-21 mice

Introduction

Radioligand imaging is a powerful in vivo method to assess the molecular basis of Alzheimer’s Disease. We therefore aimed to visualize the pathological deposition of fibrillar amyloid-β and neuronal dysfunction in aged double transgenic mice.

Methods

Using non-invasive positron emission tomography (PET) we assessed brain glucose utilization with [¹⁸F]FDG and fibrillar amyloidosis with [¹¹C]PiB and [¹⁸F]AV45 in 12 month old APPPS1-21 (n = 10) mice and their age-matched wild-type controls (n = 15). PET scans were analyzed with statistical parametric mapping (SPM) to detect significant differences in tracer uptake between genotypes. After imaging, mice were sacrificed and ex vivo measures of amyloid-β burden with immunohistochemistry as well as glucose utilization with [¹⁴C]-2DG autoradiography were obtained as gold standards.

Results

Voxel-wise SPM analysis revealed significantly decreased [¹⁸F]FDG uptake in aged APPPS1-21 mice in comparison to WT with the thalamus (96.96 %, maxT = 3.35) and striatum (61.21 %, maxT = 3.29) demonstrating the most widespread reductions at the threshold of p < 0.01. [¹¹C]PiB binding was significantly increased in APPPS1-21 mice, most notably in the hippocampus (87.84 %, maxT = 7.15) and cortex (69.08 %, maxT = 7.95), as detected by SPM voxel-wise analysis at the threshold of p < 0.01. Using the same threshold [¹⁸F]AV45 uptake was comparably lower with less significant differences. Compared to their respective ex vivo equivalents [¹⁸F]FDG demonstrated significant positive correlation to [¹⁴C]2-DG autoradiography (r = 0.67, p <0.0001) while [¹¹C]PiB and [¹⁸F]AV45 binding did not correlate to ex vivo immunohistochemistry for amyloid-β (r = 0.25, p = 0.07 and r = 0.17, p = 0.26 respectively). Lastly no correlation was observed between regions of high amyloid burden and those with decreased glucose utilization (r = 0.001, p = 0.99).

Conclusions

Our findings support that fibrillar amyloid-β deposition and reduced glucose utilization can be visualized and quantified with in vivo μPET imaging in aged APPPS1-21 mice. Therefore, the combined use of [¹⁸F]FDG and amyloid μPET imaging can shed light on the underlying relationship between fibrillar amyloid-β pathology and neuronal dysfunction.

Comparing α7 nicotinic acetylcholine receptor binding, amyloid-β deposition, and mitochondria complex-I function in living brain: A PET study in aged monkeys

This study was aimed to assess the correlations among α7 nicotinic acetylcholine receptor (α7-nAChR) binding, amyloid-β (Aβ) deposition, and mitochondrial complex I (MC-I) activity in the brain of aged monkeys (Macaca mulatta). Positron emission tomography (PET) measurements with [(11) C](R)-MeQAA, [(11) C]PIB, and [(18) F]BCPP-EF were conducted in monkeys in a conscious condition. [(11) C](R)-MeQAA binding was analyzed by a simplified reference tissue model to calculate nondisplaceable binding potential (BPND), [(11) C]PIB uptake was calculated by standard uptake value ratio (SUVR), and [(18) F]BCPP-EF binding was determined by Logan graphical analysis to calculate total distribution volume (VT) with arterial blood sampling. Higher brain uptake was determined in the thalamus, hippocampus, striatum, and cortical regions for [(11) C](R)-MeQAA, while being lower in the cerebellum. Significant age-related reduction of [(11) C](R)-MeQAA binding to α7-nAChR was determined only in the occipital cortex. The plot of Vt of [(18) F]BCPP-EF against BPND of [(11) C](R)-MeQAA indicated a significant negative correlation in the hippocampus and cortical regions in aged animals. Plotting of SUVR of [(11) C]PIB against BPND of [(11) C](R)-MeQAA showed a positive correlation. The in vivo binding of [(11) C](R)-MeQAA could reflect the upregulation of α7-nAChR induced by neurodegenerative damage determined by Aβ deposition as well as impaired MC-I activity in living brain.

The use of ¹⁸F-BCPP-EF as a PET probe for complex I activity in the brain

Mitochondria are called "cellular power plants" because they exclusively contain a respiratory electron transfer chain consisting of five components (complexes I-V) to generate most of the ATP required to maintain cellular functions. Mitochondrial complex I (MC-I) is the first and the largest macrocomplex in the pathway for oxidative phosphorylation. We recently synthesized a series of novel PET probes for quantitative imaging of MC-I activity in the living brain. Several in vitro biological evaluations suggested that (18)F-BCPP-EF could be applicable for MC-I assessment in vivo, and the probe has been applied to several animal disease models of stroke, aging, and dementia. The data suggested that (18)F-BCPP-EF could be useful to detect ischemic neuronal damage at the subacute phase, 7 days, after ischemic insult, at which time unexpectedly higher (18)F-FDG uptake was observed in the damaged area than in the contralateral intact area. Our studies with the aged monkeys demonstrated that (18)F-BCPP-EF detected the age-related reduction of MC-I activity in the living monkey brain, and also that the monkeys with higher amyloid-β deposition showed lower MC-I activity. Since PET is a sophisticated medical modality for noninvasive assessment of real-time tissue function by using target-specific radiolabeled probes, the development of novel PET probes for MC-I should be useful for diagnostic, prognostic, and treatment monitoring of diseases related to impaired MC-I function.

Differential influence of propofol and isoflurane anesthesia in a non-human primate on the brain kinetics and binding of [(18)F]DPA-714, a positron emission tomography imaging marker of glial activation

Translocator protein 18 kDa (TSPO) expression at the mitochondrial membrane of glial cells is related to glial activation. TSPO radioligands such as [(18)F]DPA-714 are useful for the non-invasive study of neuroimmune processes using positron emission tomography (PET). Anesthetic agents were shown to impact mitochondrial function and may influence [(18)F]DPA-714 binding parameters and PET kinetics. [(18) F]DPA-714 PET imaging was performed in Papio anubis baboons anesthetized using either intravenous propofol (n = 3) or inhaled isoflurane (n = 3). Brain kinetics and metabolite-corrected input function were measured to estimate [(18) F]DPA-714 brain distribution (VT). Displacement experiments were performed using PK11195 (1.5 mg/kg). In vitro [(18)F]DPA-714 binding experiments were performed using baboon brain tissue in the absence and presence of tested anesthetics. Brain radioactivity peaked higher in isoflurane-anesthetized animals compared with propofol (SUVmax = 2.7 ± 0.5 vs. 1.3 ± 0.2, respectively) but was not different after 30 min. Brain VT was not different under propofol and isoflurane. Displacement resulted in a 35.8 ± 8.4% decrease of brain radioactivity under propofol but not under isoflurane (0.1 ± 7.0%). In vitro, the presence of propofol increased TSPO density and dramatically reduced its affinity for [(18)F]DPA-714 compared with control. This in vitro effect was not significant with isoflurane. Exposure to propofol and isoflurane differentially influences TSPO interaction with its specific radioligand [(18)F]DPA-714 with subsequent impact on its tissue kinetics and specific binding estimated in vivo using PET. Therefore, the choice of anesthetics and their potential influence on PET data should be considered for the design of imaging studies using TSPO radioligands, especially in a translational research context.

[18F]DPA-714 PET imaging of translocator protein TSPO (18 kDa) in the normal and excitotoxically-lesioned nonhuman primate brain

PURPOSE

We aimed to characterize pharmacologically the TSPO- radioligand [(18)F]DPA-714 in the brain of healthy cynomolgus monkeys and evaluate the cellular origin of its binding in a model of neurodegeneration induced by intrastriatal injection of quinolinic acid (QA).

METHODS

[(18)F]DPA-714 PET images were acquired before and at 2, 7, 14, 21, 49, 70, 91 days after putaminal lesioning. Blocking and displacement studies were carried out (PK11195). Different modelling approaches estimated rate constants and V T (total distribution volume) which was used to measure longitudinal changes in the lesioned putamen. Sections for immunohistochemical labelling were prepared at the same time-points to evaluate correlations between in vivo [(18)F]DPA-714 binding and microglial/astrocytic activation.

RESULTS

[(18)F]DPA-714 showed a widespread distribution with a higher signal in the thalamus and occipital cortex and lower binding in the cerebellum. TSPO was expressed throughout the whole brain and about 73 % of [(18)F]DPA-714 binding was specific for TSPO in vivo. The one-tissue compartment model (1-TCM) provided good and reproducible estimates of V T and rate constants, and V T values from the 1-TCM and the Logan approach were highly correlated (r (2) = 0.85). QA lesioning induced an increase in V T, which was +17 %, +54 %, +157 % and +39 % higher than baseline on days 7, 14, 21 and 91 after QA injection, respectively. Immunohistochemistry revealed an early microglial and a delayed astrocytic activation after QA injection. [(18)F]DPA-714 binding matched TSPO immunopositive areas and showed a stronger colocalization with CD68 microglia than with GFAP-activated astrocytes.

CONCLUSION

[(18)F]DPA-714 binds to TSPO with high specificity in the primate brain under normal conditions and in the QA model. This tracer provides a sensitive tool for assessing neuroinflammation in the human brain.