In vivo PET Imaging of Gliogenesis After Cerebral Ischemia in Rats

In vivo imaging of cathepsin B in activated glia in the brain after orofacial formalin test

PURPOSE Cathepsin B (Cat B) is a cysteine lysosomal protease that is upregulated in many inflammatory diseases and widely expressed in the brain. Here, we used a Cat B activatable near-infrared (NIR) imaging probe to measure glial activation in vivo in the formalin test, a standard orofacial inflammatory pain model. The probe's efficacy was quantified with immunohistochemical analysis of the somatosensory cortex. PROCEDURES Three different concentrations of Cat B imaging probe (30, 50, 100 pmol/200 g bodyweight) were injected intracisternally into the foramen magnum of rats under anesthesia. Four hours later formalin (1.5%, 50 μl) was injected into the upper lip and the animal's behaviors recorded for 45 min. Subsequently, animals were repeatedly scanned using the IVIS Spectrum (8, 10, and 28 h post imaging probe injection) to measure extracellular Cat B activity. Aldehyde fixed brain sections were immunostained with antibodies against microglial marker Iba1 or astrocytic GFAP and detected with fluorescently labeled secondary antibodies to quantify co-localization with the fluorescent probe. RESULTS The Cat B imaging probe only slightly altered the formalin test results. Nocifensive behavior was only reduced in phase 1 in the 100 pmol group. In vivo measured fluorescence efficiency was highest in the 100 pmol group 28 h post imaging probe injection. Post-mortem immunohistochemical analysis of the somatosensory cortex detected the greatest amount of NIR fluorescence localized on microglia and astrocytes in the 100 pmol imaging probe group. Sensory neuron neuropeptide and cell injury marker expression in ipsilateral trigeminal ganglia was not altered by the presence of fluorescent probe. CONCLUSIONS These data demonstrate a concentration- and time-dependent visualization of extracellular Cat B in activated glia in the formalin test using a NIR imaging probe. Intracisternal injections are well suited for extracellular CNS proteinase detection in conditions when the blood-brain barrier is intact.

In vivo detection of poststroke cerebral cell proliferation in rodents and humans

OBJECTIVE

F-18-fluorothymidine (FLT) is a positron emission tomography (PET) tracer for imaging cell proliferation in vivo. We aimed to assess FLT uptake as a marker for cerebral cell proliferation in a rat model of ischemic stroke and patients with cerebral infarct, correlating with disease severity and outcomes.

METHODS

Cerebral FLT PET was performed in rats subjected to transient middle cerebral artery occlusion (MCAO) and patients with cerebral infarct. PET data were analyzed and expressed as average standardized uptake value ratios (SUVRs) using cerebellar cortex as reference. Infarct volume was analyzed by 2,3,5-triphenyltetrazolium chloride staining in rats and by magnetic resonance imaging in patients. Neurological function was assessed using modified Neurological Severity Score (mNSS) for rats and National Institutes of Health Stroke Scale (NIHSS) for patients.

RESULTS

Seven days post-MCAO, rats' FLT PET displayed higher SUVRs in the infarcted brain, declining gradually until Day 28. FLT-binding ratio (SUVR in the infarcted brain divided by that in contralateral side) correlated positively with stroke severity (p < 0.001), and to early mNSS decline in rats with mild to moderate stroke severity (p = 0.031). In 13 patients with cerebral infarct, FLT PET showed high SUVR in the infarcted regions. FLT-binding ratio correlated positively with infarct volume (p = 0.006). Age-adjusted initial NIHSS (p = 0.035) and early NIHSS decline (p = 0.076) showed significance or a trend toward positive correlation with the FLT-binding ratio.

INTERPRETATION

In vivo FLT PET detects poststroke cerebral cell proliferation, which is associated with stroke severity and/or outcomes in MCAO rats and patients with cerebral infarct.

Non-invasive in vivo imaging of brain and retinal microglia in neurodegenerative diseases

Microglia play crucial roles in immune responses and contribute to fundamental biological processes within the central nervous system (CNS). In neurodegenerative diseases, microglia undergo functional changes and can have both protective and pathogenic roles. Microglia in the retina, as an extension of the CNS, have also been shown to be affected in many neurological diseases. While our understanding of how microglia contribute to pathological conditions is incomplete, non-invasive in vivo imaging of brain and retinal microglia in living subjects could provide valuable insights into their role in the neurodegenerative diseases and open new avenues for diagnostic biomarkers. This mini-review provides an overview of the current brain and retinal imaging tools for studying microglia in vivo. We focus on microglia targets, the advantages and limitations of in vivo microglia imaging approaches, and applications for evaluating the pathogenesis of neurological conditions, such as Alzheimer's disease and multiple sclerosis.

Oxygen-glucose deprivation-induced glial cell reactivity in the rat primary neuron-glia co-culture

It has been demonstrated that in vivo brain ischemia induces activation and proliferation of astrocytes and microglia. However, the mechanism underlying the ischemia-induced activation and proliferation of these cells remains to be unclear. Oxygen-glucose deprivation (OGD), an in vitro ischemia mimic, has been extensively used to analyze the hypoxia response of various cell types. This study examined the OGD-induced changes in the expression level of astrocytes and microglia marker proteins and immunoreactivity for Ki-67, a marker protein for cell proliferation, using rat primary hippocampal neuron-glia co-culture (NGC) cells. Furthermore, OGD-induced changes in the expression of M1/M2 microglia phenotype-related genes were also examined. MTT assay indicated that 120 min of OGD decreased cell viability, and immunocytochemistry indicated that 120 min of OGD abolished most microtubule-associated protein 2 (MAP2)-immunopositive neurons. In contrast, glial fibrillary acidic protein (GFAP)-immunopositive astrocytes and ionized calcium-binding adapter protein-1 (Iba-1)-immunopositive microglia, and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase)-immunopositive oligodendrocytes survived OGD. Western blot assays and double-immunofluorescent staining indicated that OGD increased the GFAP expression level and the Ki-67-immunopositive/GFAP-immunopositive cells' ratio. Real-time PCR analysis showed that OGD altered M1 microglia phenotype-related genes. Specifically, OGD decreased the expression level of CD32 and interleukin-1β (IL-1β) genes and increased that of the inducible nitric oxide synthase (iNOS) gene. Therefore, applying OGD to NGC cells could serve as a useful in vitro tool to elucidate the molecular mechanisms underlying brain ischemia-induced changes in GFAP expression, astrocyte proliferation, and M1 microglia phenotype-related gene expression.

Changes in Brain Electrical Activity after Transient Middle Cerebral Artery Occlusion in Rats

Objectives. Ischemic stroke is a leading cause of death and disability worldwide. To search for new therapeutic and pharmacotherapeutic strategies, numerous models of this disease have been proposed, the most popular being transient middle cerebral artery occlusion. Behavioral and sensorimotor testing, biochemical, and histological methods are traditionally used in conjunction with this model to assess the effectiveness of potential treatment options. Despite its wide overall popularity, electroencephalography/electrocorticography is quite rarely used in such studies. Materials and methods. In the present work, we explored the changes in brain electrical activity at days 3 and 7 after 30- and 45-min of transient middle cerebral artery occlusion in rats. Results. Cerebral ischemia altered the amplitude and spectral electrocorticogram characteristics, and led to a reorganization of inter- and intrahemispheric functional connections. Ischemia duration affected the severity as well as the nature of the observed changes. Conclusions. The dynamics of changes in brain electrical activity may indicate a spontaneous partial recovery of impaired cerebral functions at post-surgery day 7. Our results suggest that electrocorticography can be used successfully to assess the functional status of the brain following ischemic stroke in rats as well as to investigate the dynamics of functional recovery.

Neuroinflammation, Stroke, Blood-Brain Barrier Dysfunction, and Imaging Modalities

Maintaining blood-brain barrier (BBB) integrity is crucial for the homeostasis of the central nervous system. Structurally comprising the BBB, brain endothelial cells interact with pericytes, astrocytes, neurons, microglia, and perivascular macrophages in the neurovascular unit. Brain ischemia unleashes a profound neuroinflammatory response to remove the damaged tissue and prepare the brain for repair. However, the intense neuroinflammation occurring during the acute phase of stroke is associated with BBB breakdown, neuronal injury, and worse neurological outcomes. Here, we critically discuss the role of neuroinflammation in ischemic stroke pathology, focusing on the BBB and the interactions between central nervous system and peripheral immune responses. We highlight inflammation-driven injury mechanisms in stroke, including oxidative stress, increased MMP (matrix metalloproteinase) production, microglial activation, and infiltration of peripheral immune cells into the ischemic tissue. We provide an updated overview of imaging techniques for in vivo detection of BBB permeability, leukocyte infiltration, microglial activation, and upregulation of cell adhesion molecules following ischemic brain injury. We discuss the possibility of clinical implementation of imaging modalities to assess stroke-associated neuroinflammation with the potential to provide image-guided diagnosis and treatment. We summarize the results from several clinical studies evaluating the efficacy of anti-inflammatory interventions in stroke. Although convincing preclinical evidence suggests that neuroinflammation is a promising target for ischemic stroke, thus far, translating these results into the clinical setting has proved difficult. Due to the dual role of inflammation in the progression of ischemic damage, more research is needed to mechanistically understand when the neuroinflammatory response begins the transition from injury to repair. This could have important implications for ischemic stroke treatment by informing time- and context-specific therapeutic interventions.

PET imaging of reactive astrocytes in neurological disorders

The reactive astrocytes manifest molecular, structural, and functional remodeling in injury, infection, or diseases of the CNS, which play a critical role in the pathological mechanism of neurological diseases. A growing need exists for dependable approach to better characterize the activation of astrocyte in vivo. As an advanced molecular imaging technology, positron emission tomography (PET) has the potential for visualizing biological activities at the cellular levels. In the review, we summarized the PET visualization strategies for reactive astrocytes and discussed the applications of astrocyte PET imaging in neurological diseases. Future studies are needed to pay more attention to the development of specific imaging agents for astrocytes and further improve our exploration of reactive astrocytes in various diseases.

Research progress of stem cell therapy for ischemic stroke

Ischemic stroke is a serious cerebrovascular disease with high morbidity, disability and mortality. There is no doubt that the disease has a severe impact on the physical and mental health and quality of life of patients, as well as impose a heavy burden on families and societies. Unfortunately, there has been a lack of effective treatment. This overview reviews the pathophysiology of stem cell therapy in Ischemic stroke, and discuss its effects on neurogenesis, the latest clinical trials, and advances in tracking and monitoring of endogenous and exogenous stem cells.

In vivo multimodal imaging of adenosine A1 receptors in neuroinflammation after experimental stroke

Adenosine A₁ receptors (A₁ARs) are promising imaging biomarkers and targets for the treatment of stroke. Nevertheless, the role of A₁ARs on ischemic damage and its subsequent neuroinflammatory response has been scarcely explored so far.

Methods: In this study, the expression of A₁ARs after transient middle cerebral artery occlusion (MCAO) was evaluated by positron emission tomography (PET) with [¹⁸F]CPFPX and immunohistochemistry (IHC). In addition, the role of A₁ARs on stroke inflammation using pharmacological modulation was assessed with magnetic resonance imaging (MRI), PET imaging with [¹⁸F]DPA-714 (TSPO) and [¹⁸F]FLT (cellular proliferation), as well as IHC and neurofunctional studies.

Results: In the ischemic territory, [¹⁸F]CPFPX signal and IHC showed the overexpression of A₁ARs in microglia and infiltrated leukocytes after cerebral ischemia. Ischemic rats treated with the A₁AR agonist ENBA showed a significant decrease in both [¹⁸F]DPA-714 and [¹⁸F]FLT signal intensities at day 7 after cerebral ischemia, a feature that was confirmed by IHC results. Besides, the activation of A₁ARs promoted the reduction of the brain lesion, as measured with T₂W-MRI, and the improvement of neurological outcome including motor, sensory and reflex responses. These results show for the first time the in vivo PET imaging of A₁ARs expression after cerebral ischemia in rats and the application of [¹⁸F]FLT to evaluate glial proliferation in response to treatment.

Conclusion: Notably, these data provide evidence for A₁ARs playing a key role in the control of both the activation of resident glia and the de novo proliferation of microglia and macrophages after experimental stroke in rats.