Imaging of reactive oxygen species in focal ischemic mouse brain using a radical trapping tracer [(3)H]hydromethidine

[123I]CLINDE SPECT as a neuroinflammation imaging approach in a rat model of stroke

Poststroke neuroinflammation exacerbates disease progression. [11C]PK11195-positron emission tomography (PET) imaging has been used to visualize neuroinflammation; however, its short half-life of 20 min limits its clinical use. [123I]CLINDE has a longer half-life (13h); therefore, [123I]CLINDE-single-photon emission computed tomography (SPECT) imaging is potentially more practical than [11C]PK11195-PET imaging in clinical settings. The objectives of this study were to 1) validate neuroinflammation imaging using [123I]CLINDE and 2) investigate the mechanisms underlying stroke in association with neuroinflammation using multimodal techniques, including magnetic resonance imaging (MRI), gas-PET, and histological analysis, in a rat model of ischemic stroke, that is, permanent middle cerebral artery occlusion (pMCAo). At 6 days post-pMCAo, [123I]CLINDE-SPECT considerably corresponded to the immunohistochemical images stained with the CD68 antibody (a marker for microglia/microphages), comparable to the level observed in [11C]PK11195-PET images. In addition, the [123I]CLINDE-SPECT images corresponded well with autoradiography images. Rats with severe infarcts, as defined by MRI, exhibited marked neuroinflammation in the peri-infarct area and less neuroinflammation in the ischemic core, accompanied by a substantial reduction in the cerebral metabolic rate of oxygen (CMRO2) in 15O-gas-PET. Rats with moderate-to-mild infarcts exhibited neuroinflammation in the ischemic core, where CMRO2 levels were mildly reduced. This study demonstrates that [123I]CLINDE-SPECT imaging is suitable for neuroinflammation imaging and that the distribution of neuroinflammation varies depending on the severity of infarction.

Imaging of microglia in post-stroke inflammation

Microglia constantly survey the central nervous system microenvironment and maintain brain homeostasis. Microglia activation, polarization and inflammatory response are of great importance in the pathophysiology of ischemic stroke. For exploring biochemical processes in vivo, positron emission tomography (PET) is a superior imaging tool. Translocator protein 18 kDa (TSPO), is a validated neuroinflammatory biomarker which is widely used to evaluate various central nervous system (CNS) pathologies in both preclinical and clinical studies. TSPO level can be elevated due to peripheral inflammatory cells infiltration and glial cells activation. Therefore, a clear understanding of the dynamic changes between microglia and TSPO is critical for interpreting PET studies and understanding the pathophysiology after ischemic stroke. Our review discusses alternative biological targets that have attracted considerable interest for the imaging of microglia activation in recent years, and the potential value of imaging of microglia in the assessment of stroke therapies.

Mechanistic and therapeutic role of Drp1 in the pathogenesis of stroke

Stroke had emerged as one of the leading causes of death and long-term disability across the globe. Emerging evidence suggests a significant increase in the incidence of stroke with age, which is further expected to increase dramatically owing to an ever-expanding elderly population. The current situation imposes a significant burden on the healthcare system and requires a deeper understanding of the underlying mechanisms and development of novel interventions. It is well established that mitochondrial dysfunction plays a pivotal role in the onset of stroke. Dynamin-related protein 1 (Drp1), is a key regulator of mitochondria fission, and plays a crucial role during the pathogenesis of stroke. Drp1 protein levels significantly increase after stroke potentially in a p38 mitogen-activated protein kinases (MAPK) dependent manner. Protein phosphatase 2A (PP2A) facilitate mitochondrial fission and cell death by dephosphorylating the mitochondrial fission enzyme Drp1 at the inhibitory phosphorylation site serine 637. Outer mitochondrial membrane A-Kinase Anchoring Proteins 1 (AKAP 1) and protein kinase A complex (PKA) complex inhibits Drp1-dependent mitochondrial fission by phosphorylating serine 637. Drp1 activation promotes the release of cytochrome C from mitochondria and therefore leads to apoptosis. In addition, Drp1 activation inhibits mitochondrial glutathione dependent free radical scavenging, which further enhances the ROS level and exacerbate mitochondrial dysfunction. Drp1 translocate p53 to mitochondrial membrane and leads to mitochondria-related necrosis. The current review article discusses the possible mechanistic pathways by which Drp1 can influence the pathogenesis of stroke. Besides, it will describe various inhibitors for Drp1 and their potential role as therapeutics for stroke in the future.

Neuroprotective efficacy of N-t-butylhydroxylamine (NtBHA) in transient focal ischemia in rats

The pharmacological or toxicological activities of the degradation products of drug candidates have been unaddressed during the drug development process. Ischemic stroke accounts for 80% of all strokes and is responsible for considerable mortality and disability worldwide. Despite decades of research on neuroprotective agents, tissue plasminogen activators (t-PA), a thrombolytic agent, remains the only approved acute stroke pharmacological therapy. NXY-059, a free radical scavenger, exhibited striking neuroprotective properties in preclinical models and met all the criteria established by the Stroke Academic Industry Roundtable (STAIR) for a neuroprotective agent. In phase 3 clinical trials, NXY-059 exhibited significant neuroprotective effects in one trial (SAINT-I), but not in the second (SAINT-II). Some have hypothesized that N-t-butyl hydroxylamine (NtBHA), a breakdown product of NXY-059 was the actual neuroprotective agent in SAINT-I and that changes to the formulation of NXY-059 to prevent its breakdown to NtBHA in SAINT -II was the reason for the lack of efficacy. We evaluated the neuroprotective effect of NtBHA in N-methyl-D-aspartate (NMDA)-treated primary neurons and in rat focal cerebral ischemia. NtBHA significantly attenuated infarct volume in rat transient focal ischemia, and attenuated NMDA-induced cytotoxicity in primary cortical neurons. NtBHA also reduced free radical generation and exhibited mitochondrial protection.

Radiotracers for Nuclear Imaging of Reactive Oxygen Species: Advances Made So Far

Reactive oxygen species (ROS) are a cluster of highly reactive and short-lived oxygen-containing molecules that lead to metabolic disorders where production exceeds catabolism in an organism. Many specific radiotracers for positron/single-photon emission tomography have been developed to reveal the discrepancy of ROS levels in normal and damaged tissues and further clarify the relationship between ROS and diseases. This review summarizes the advances achieved for the development of ROS radiotracers to date. The structure design, radiosynthesis, and imaging performance of existing radiotracers are discussed with the individual ROS-response mechanisms highlighted.

Redox Monitoring in Nuclear Medical Imaging

Significance: The imbalance in redox homeostasis is known as oxidative stress, which is relevant to many diseases such as cancer, arteriosclerosis, and neurodegenerative disorders. Overproduction of reactive oxygen species (ROS) is one of the factors that trigger the redox state imbalance in vivo. The ROS have high reactivity and impair biomolecules, whereas antioxidants and antioxidant enzymes, such as ascorbate and glutathione, reduce the overproduction of ROS to rectify the redox imbalance. Owing to this, redox monitoring tools have been developed to understand the redox fluctuations in oxidative stress-related diseases. Recent Advances: In an attempt to monitor redox substances, including ROS and radical species, versatile modalities have been developed, such as electron spin resonance, chemiluminescence, and fluorescence. In particular, many fluorescent probes have been developed that are selective for ROS. This has significantly contributed to understanding the relevance of ROS in disease onset and progression. Critical Issues: To date, the dynamics of ROS and radical fluctuation in in vivo redox states remain unclear, and there are a few methods for the in vivo detection of redox fluctuations. Future Directions: In this review, we summarize the development of radiolabeled probes for monitoring redox-relevant species by nuclear medical imaging that is applicable in vivo. In the future, translational research is likely to be advanced through the development of highly sensitive and in vivo applicable detection methods, such as nuclear medical imaging, to clarify the underlying dynamics of ROS, radicals, and redox substances in many diseases. Antioxid. Redox Signal. 36, 797-810.

Development of a Positron Emission Tomography Radiotracer for Imaging Elevated Levels of Superoxide in Neuroinflammation

Reactive oxygen species (ROS) are believed to play a major role in the proinflammatory, M1-polarized form of neuroinflammation. However, it has been difficult to assess the role of ROS and their role in neuroinflammation in animal models of disease because of the absence of probes capable of measuring their presence with the functional imaging technique positron emission tomography (PET). This study describes the synthesis and in vivo evaluation of [¹⁸F]ROStrace, a radiotracer for imaging superoxide in vivo with PET, in an LPS model of neuroinflammation. [¹⁸F]ROStrace was found to rapidly cross the blood–brain barrier (BBB) and was trapped in the brain of LPS-treated animals but not the control group. [¹⁸F]ox-ROStrace, the oxidized form of [¹⁸F]ROStrace, did not cross the BBB. These data suggest that [¹⁸F]ROStrace is a suitable radiotracer for imaging superoxide levels in the central nervous system with PET.

PET Imaging of Microglial Activation-Beyond Targeting TSPO

Neuroinflammation, which involves microglial activation, is thought to play a key role in the development and progression of neurodegenerative diseases and other brain pathologies. Positron emission tomography is an ideal imaging technique for studying biochemical processes in vivo, and particularly for studying the living brain. Neuroinflammation has been traditionally studied using radiotracers targeting the translocator protein 18 kDa, but this comes with certain limitations. The current review describes alternative biological targets that have gained interest for the imaging of microglial activation over recent years, such as the cannabinoid receptor type 2, cyclooxygenase-2, the P2X₇ receptor and reactive oxygen species, and some promising radiotracers for these targets. Although many advances have been made in the field of neuroinflammation imaging, current radiotracers all target the pro-inflammatory (M1) phenotype of activated microglia, since the number of known biological targets specific for the anti-inflammatory (M2) phenotype that are also suited as a target for radiotracer development is still limited. Next to proceeding the currently available tracers for M1 microglia into the clinic, the development of a suitable radiotracer for M2 microglia would mean a great advance in the field, as this would allow for imaging of the dynamics of microglial activation in different diseases.

Neurotransmitters in the mediation of cerebral ischemic injury

Under physiological conditions, neurotransmitters shape neuronal networks and control several cellular and synaptic functions. In the mammalian central nervous system (CNS), excitatory and inhibitory neurotransmission are mediated in large part by glutamate and gamma-aminobutyric acid (GABA), which are excitatory and inhibitory neurotransmitters, respectively. Glutamate and GABA also play crucial roles in neurological disorders such as cerebral ischemia. Glutamate in particular causes excitotoxicity, known as one of the hallmark mechanisms in the pathophysiology of cerebral ischemic injury for more than thirty years. Excitotoxicity occurs due to excessive glutamate release leading to overactivation of postsynaptic glutamate receptors, which evokes a downstream cascade that eventually leads to neuronal dysfunction and degeneration. Also, a reduction in GABA receptor response after ischemia impedes these inhibitory effectors from attenuating excitotoxicity and thereby further enabling the excitotoxic insult. This review focuses on the mechanisms by which glutamate and GABA mediate excitotoxicity and ischemic injury. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Evaluation of a novel radiotracer for positron emission tomography imaging of reactive oxygen species in the central nervous system

INTRODUCTION

Few, if any, radiotracers are available for the in vivo imaging of reactive oxygen species (ROS) in the central nervous system. ROS play a critical role in normal cell processes such as signaling and homeostasis but overproduction of ROS is implicated in several disorders. We describe here the radiosynthesis and initial ex vivo and in vivo evaluation of [¹¹C]hydromethidine ([¹¹C]HM) as a radiotracer to image ROS using positron emission tomography (PET).

METHODS

[¹¹C]HM and its deuterated isotopologue [¹¹C](4) were produced using [¹¹C]methyl triflate in a one-pot, two-step reaction and purified by high performance liquid chromatography. Ex vivo biodistribution studies were performed after tail vein injections of both radiotracers. To demonstrate sensitivity of uptake to ROS, [¹¹C]HM was administered to rats treated systemically with lipopolysaccharide (LPS). In addition, ex vivo autoradiography and in vivo PET imaging were performed using [¹¹C]HM on rats which had been microinjected with sodium nitroprusside (SNP) to induce ROS.

RESULTS

[¹¹C]HM and [¹¹C](4) radiosyntheses were reliable and produced the radiotracers at high specific activities and radiochemical purities. Both radiotracers demonstrated good brain uptake and fast washout of radioactivity, but [¹¹C](4) washout was faster. Pretreatment with LPS resulted in a significant increase in brain retention of radioactivity. Ex vivo autoradiography and PET imaging of rats unilaterally treated with microinjections of SNP demonstrated increased retention of radioactivity in the treated side of the brain.

CONCLUSIONS

[¹¹C]HM has the attributes of a radiotracer for PET imaging of ROS in the brain including good brain penetration and increased retention of radioactivity in animal models of oxidative stress.

Multimodal Imaging of Neurometabolic Pathology due to Traumatic Brain Injury

The impact of traumatic brain injury (TBI) involves a combination of complex biochemical processes beginning with the initial insult and lasting for days, months and even years post-trauma. These changes range from neuronal integrity losses to neurotransmitter imbalance and metabolite dysregulation, leading to the release of pro- or anti-apoptotic factors which mediate cell survival or death. Such dynamic processes affecting the brain's neurochemistry can be monitored using a variety of neuroimaging techniques, whose combined use can be particularly useful for understanding patient-specific clinical trajectories. Here, we describe how TBI changes the metabolism of essential neurochemical compounds, summarize how neuroimaging approaches facilitate the study of such alterations, and highlight promising ways in which neuroimaging can be used to investigate post-TBI changes in neurometabolism.

Targeting oxidative stress for the treatment of ischemic stroke: Upstream and downstream therapeutic strategies

Excessive oxygen and its chemical derivatives, namely reactive oxygen species (ROS), produce oxidative stress that has been known to lead to cell injury in ischemic stroke. ROS can damage macromolecules such as proteins and lipids and leads to cell autophagy, apoptosis, and necrosis to the cells. This review describes studies on the generation of ROS, its role in the pathogenesis of ischemic stroke, and recent development in therapeutic strategies in reducing oxidative stress after ischemic stroke.