Characterization of a novel murine model for spontaneous hemorrhagic stroke using in vivo PET and MR multiparametric imaging

Blood-Brain Barrier Disruption and Imaging Assessment in Stroke

Disruption of the blood-brain barrier (BBB) is an important pathological hallmark of ischemic stroke. Blood-brain barrier disruption (BBBD) is a consequence of ischemia and may also exacerbate damage to brain parenchyma. Therefore, maintaining BBB integrity is critical for the central nervous system (CNS) homeostasis. This review offers a concise overview of BBB structure and function, along with the mechanisms underlying its impairment following a stroke. In addition, we review the recent imaging techniques employed to study blood-brain barrier permeability (BBBP) in the context of ischemic brain injury with the goal of providing imaging guidance for stroke diagnosis and treatment from the perspective of the BBBD. This knowledge is vital for developing strategies to safeguard the BBB during cerebral ischemia.

Cerebrovascular Gi Proteins Protect Against Brain Hypoperfusion and Collateral Failure in Cerebral Ischemia

Cerebral hypoperfusion and vascular dysfunction are closely related to common risk factors for ischemic stroke such as hypertension, dyslipidemia, diabetes, and smoking. The role of inhibitory G protein-dependent receptor (G_iPCR) signaling in regulating cerebrovascular functions remains largely elusive. We examined the importance of G_iPCR signaling in cerebral blood flow (CBF) and its stability after sudden interruption using various in vivo high-resolution magnetic resonance imaging techniques. To this end, we induced a functional knockout of G_iPCR signaling in the brain vasculature by injection of pertussis toxin (PTX). Our results show that PTX induced global brain hypoperfusion and microvascular collapse. When PTX-pretreated animals underwent transient unilateral occlusion of one common carotid artery, CBF was disrupted in the ipsilateral hemisphere resulting in the collapse of the cortically penetrating microvessels. In addition, pronounced stroke features in the affected brain regions appeared in both MRI and histological examination. Our findings suggest an impact of cerebrovascular G_iPCR signaling in the maintenance of CBF, which may be useful for novel pharmacotherapeutic approaches to prevent and treat cerebrovascular dysfunction and stroke.

Repurposing the mucolytic agent ambroxol for treatment of sub-acute and chronic ischaemic stroke

Ambroxol is a well-known mucolytic expectorant, which has gained much attention in amyotrophic lateral sclerosis, Parkinson's and Gaucher's disease. A specific focus has been placed on ambroxol's glucocerebrosidase-stimulating activity, on grounds that the point mutation of the gba1 gene, which codes for this enzyme, is a risk factor for developing Parkinson's disease. However, ambroxol has been attributed other characteristics, such as the potent inhibition of sodium channels, modification of calcium homeostasis, anti-inflammatory effects and modifications of oxygen radical scavengers. We hypothesized that ambroxol could have a direct impact on neuronal rescue if administered directly after ischaemic stroke induction. We longitudinally evaluated 53 rats using magnetic resonance imaging to examine stroke volume, oedema, white matter integrity, resting state functional MRI and behaviour for 1 month after ischemic stroke onset. For closer mechanistic insights, we evaluated tissue metabolomics of different brain regions in a subgroup of animals using ex vivo nuclear magnetic resonance spectroscopy. Ambroxol-treated animals presented reduced stroke volumes, reduced cytotoxic oedema, reduced white matter degeneration, reduced necrosis, improved behavioural outcomes and complex changes in functional brain connectivity. Nuclear magnetic resonance spectroscopy tissue metabolomic data at 24 h post-stroke proposes several metabolites that are capable of minimizing post-ischaemic damage and that presented prominent shifts during ambroxol treatment in comparison to controls. Taking everything together, we propose that ambroxol catalyzes recovery in energy metabolism, cellular homeostasis, membrane repair mechanisms and redox balance. One week of ambroxol administration following stroke onset reduced ischaemic stroke severity and improved functional outcome in the subacute phase followed by reduced necrosis in the chronic stroke phase.

Multiparametric Longitudinal Profiling of RCAS-tva-Induced PDGFB-Driven Experimental Glioma

Glioblastomas are incurable primary brain tumors harboring a heterogeneous landscape of genetic and metabolic alterations. Longitudinal imaging by MRI and [18F]FET-PET measurements enable us to visualize the features of evolving tumors in a dynamic manner. Yet, close-meshed longitudinal imaging time points for characterizing temporal and spatial metabolic alterations during tumor evolution in patients is not feasible because patients usually present with already established tumors. The replication-competent avian sarcoma-leukosis virus (RCAS)/tumor virus receptor-A (tva) system is a powerful preclinical glioma model offering a high grade of spatial and temporal control of somatic gene delivery in vivo. Consequently, here, we aimed at using MRI and [18F]FET-PET to identify typical neuroimaging characteristics of the platelet-derived growth factor B (PDGFB)-driven glioma model using the RCAS-tva system. Our study showed that this preclinical glioma model displays MRI and [18F]FET-PET features that highly resemble the corresponding established human disease, emphasizing the high translational relevance of this experimental model. Furthermore, our investigations unravel exponential growth dynamics and a model-specific tumor microenvironment, as assessed by histology and immunochemistry. Taken together, our study provides further insights into this preclinical model and advocates for the imaging-stratified design of preclinical therapeutic interventions.

Formic acid induces hypertension-related hemorrhage in hSSAOTG in mice and human

Hypertension is a confirmed risk factor for cerebral hemorrhage in humans. Which endogenous factor directly induces hypertension-related hemorrhage is unclear. In this study, 42 hemorrhagic patients with hypertension and hyperlipidemia and 42 age-matched healthy controls were enrolled. The contents of serum semicarbazide-sensitive amine oxidase (SSAO) and formic acid (FC, FC is a final product of SSAO through the oxidation of endogenous formaldehyde, which results from the enzymatic oxidative deamination of the SSAO substrate, methylamine) were examined in the patients after stroke. Hemorrhagic areas were quantified by computer tomography. In the animal study, hemorrhagic degree was assessed by hemotoxylin & eosin or tissue hemoglobin kits. The relationship between FC and blood pressure/hemorrhagic degree was examined in wild-type mice and hSSAO^TG mice fed with high-fat diets or high-fat and -salt diets. The results showed that the levels of serum FC were positively correlated with blood pressure and hemorrhagic areas in hemorrhagic patients. Transfection of microRNA-134 could enhance SSAO expression in human vascular smooth muscle cells. Consistently, after treatment with high-fat and -salt diets, hSSAO^TG mice exhibited higher levels of miR134 and FC, higher blood pressure, and more severe hemorrhage than wild-type mice. Interestingly, folic acid reduced hypertension and hemorrhage in hSSAO^TG mice fed with high-fat diets. These findings suggest that FC is a crucial endogenous factor for hypertension and hemorrhage.

Assessing the Evolution of Intracranial Hematomas by using Animal Models: A Review of the Progress and the Challenges

Stroke has become the second leading cause of death in people aged higher than 60 years, with cancer being the first. Intracerebral hemorrhage (ICH) is the most lethal type of stroke. Using imaging techniques to evaluate the evolution of intracranial hematomas in patients with hemorrhagic stroke is worthy of ongoing research. The difficulty in obtaining ultra-early imaging data and conducting intensive dynamic radiographic imaging in actual clinical settings has led to the application of experimental animal models to assess the evolution of intracranial hematomas. Herein, we review the current knowledge on primary intracerebral hemorrhage mechanisms, focus on the progress of animal studies related to hematoma development and secondary brain injury, introduce preclinical therapies, and summarize related challenges and future directions.

Machine learning identifies stroke features between species

Identification and localization of ischemic stroke (IS) lesions is routinely performed to confirm diagnosis, assess stroke severity, predict disability and plan rehabilitation strategies using magnetic resonance imaging (MRI). In basic research, stroke lesion segmentation is necessary to study complex peri-infarction tissue changes. Moreover, final stroke volume is a critical outcome evaluated in clinical and preclinical experiments to determine therapy or intervention success. Manual segmentations are performed but they require a specialized skill set, are prone to inter-observer variation, are not entirely objective and are often not supported by histology. The task is even more challenging when dealing with large multi-center datasets, multiple experimenters or large animal cohorts. On the other hand, current automatized segmentation approaches often lack histological validation, are not entirely user independent, are often based on single parameters, or in the case of complex machine learning methods, require vast training datasets and are prone to a lack of model interpretation.

Methods: We induced IS using the middle cerebral artery occlusion model on two rat cohorts. We acquired apparent diffusion coefficient (ADC) and T2-weighted (T2W) images at 24 h and 1-week after IS induction. Subsets of the animals at 24 h and 1-week post IS were evaluated using histology and immunohistochemistry. Using a Gaussian mixture model, we segmented voxel-wise interactions between ADC and T2W parameters at 24 h using one of the rat cohorts. We then used these segmentation results to train a random forest classifier, which we applied to the second rat cohort. The algorithms' stroke segmentations were compared to manual stroke delineations, T2W and ADC thresholding methods and the final stroke segmentation at 1-week. Volume correlations to histology were also performed for every segmentation method. Metrics of success were calculated with respect to the final stroke volume. Finally, the trained random forest classifier was tested on a human dataset with a similar temporal stroke on-set. Manual segmentations, ADC and T2W thresholds were again used to evaluate and perform comparisons with the proposed algorithms' output.

Results: In preclinical rat data our framework significantly outperformed commonly applied automatized thresholding approaches and segmented stroke regions similarly to manual delineation. The framework predicted the localization of final stroke regions in 1-week post-stroke MRI with a median Dice similarity coefficient of 0.86, Matthew's correlation coefficient of 0.80 and false positive rate of 0.04. The predicted stroke volumes also strongly correlated with final histological stroke regions (Pearson correlation = 0.88, P < 0.0001). Lastly, the stroke region characteristics identified by our framework in rats also identified stroke lesions in human brains, largely outperforming thresholding approaches in stroke volume prediction (P<0.01).

Conclusion: Our findings reveal that the segmentation produced by our proposed framework using 24 h MRI rat data strongly correlated with the final stroke volume, denoting a predictive effect. In addition, we show for the first time that the stroke imaging features can be directly translated between species, allowing identification of acute stroke in humans using the model trained on animal data. This discovery reduces the gap between the clinical and preclinical fields, unveiling a novel approach to directly co-analyze clinical and preclinical data. Such methods can provide further biological insights into human stroke and highlight the differences between species in order to help improve the experimental setups and animal models of the disease.

Selective protection of murine cerebral Gi/o-proteins from inactivation by parenterally injected pertussis toxin

Pertussis toxin (PTX) is a potent virulence factor in patients suffering from whooping cough, but in its detoxified version, it is applied for vaccination. It is thought to contribute to the pathology of the disease including various CNS malfunctions. Based on its enzymatic activity, PTX disrupts GPCR-dependent signaling by modifying the α-subunit of heterotrimeric G_i/o-proteins. It is also extensively used as a research tool to study neuronal functions in vivo and in vitro. However, data demonstrating the penetration of PTX from the blood into the brain are missing. Here, we examined the Gα_i/o-modifying activity of PTX in murine brains after its parenteral application. Ex vivo biodistribution analysis of [¹²⁴I]-PTX displayed poor distribution to the brain while relatively high concentrations were visible in the pancreas. PTX affected CNS and endocrine functions of the pancreas as shown by open-field and glucose tolerance tests, respectively. However, while pancreatic islet Gα_i/o-proteins were modified, their neuronal counterparts in brain tissue were resistant towards PTX as indicated by different autoradiographic and immunoblot SDS-PAGE analyses. In contrast, PTX easily modified brain Gα_i/o-proteins ex vivo. An attempt to increase BBB permeability by application of hypertonic mannitol did not show PTX activity on neuronal G proteins. Consistent with these findings, in vivo MRI analysis did not point to an increased blood-brain barrier (BBB) permeability following PTX treatment. Our data demonstrate that the CNS is protected from PTX. Thus, we hypothesize that the BBB hinders PTX to penetrate into the CNS and to deliver its enzymatic activity to brain Gα_i/o-proteins. KEY MESSAGES: i.p. applied PTX is poorly retained in the brain while reaches high concentration in the pancreas. Pancreatic islet Gα_i/o- but not cerebral Gα_i/o-proteins are modified by i.p. administered PTX. Gα_i/o-proteins from isolated cerebral cell membranes were easily modified by PTX ex vivo. CNS is protected from i.p. administered PTX. PTX does not permeabilize the BBB.

PET/MRI Hybrid Systems

Over the last decade, the combination of PET and MRI in one system has proven to be highly successful in basic preclinical research, as well as in clinical research. Nowadays, PET/MRI systems are well established in preclinical imaging and are progressing into clinical applications to provide further insights into specific diseases, therapeutic assessments, and biological pathways. Certain challenges in terms of hardware had to be resolved concurrently with the development of new techniques to be able to reach the full potential of both combined techniques. This review provides an overview of these challenges and describes the opportunities that simultaneous PET/MRI systems can exploit in comparison with stand-alone or other combined hybrid systems. New approaches were developed for simultaneous PET/MRI systems to correct for attenuation of 511 keV photons because MRI does not provide direct information on gamma photon attenuation properties. Furthermore, new algorithms to correct for motion were developed, because MRI can accurately detect motion with high temporal resolution. The additional information gained by the MRI can be employed to correct for partial volume effects as well. The development of new detector designs in combination with fast-decaying scintillator crystal materials enabled time-of-flight detection and incorporation in the reconstruction algorithms. Furthermore, this review lists the currently commercially available systems both for preclinical and clinical imaging and provides an overview of applications in both fields. In this regard, special emphasis has been placed on data analysis and the potential for both modalities to evolve with advanced image analysis tools, such as cluster analysis and machine learning.

Impact of the Arterial Input Function Recording Method on Kinetic Parameters in Small-Animal PET

The goal of this study was to validate the use of an MR-compatible blood sampler (BS) with a detector system based on a lutetium oxyorthosilicate scintillator and avalanche photodiodes for small-animal PET. Methods: Five rats underwent a 60-min ¹⁸F-FDG study. For each animal, the arterial input function (AIF) was derived from the BS recording, from manual sampling (MS), and from the PET image. These AIFs were applied for kinetic modeling of the striatum using the irreversible 2-tissue-compartment model. The MS-based technique with a dispersion correction served as a reference approach, and the kinetic parameters that were estimated with the BS- and the image-derived AIFs were compared with the reference values. Additionally, the effect of applying a population-based activity ratio for plasma to whole blood (p/wb) and the dispersion correction was assessed. Results: The K₁, k₂, and k₃ values estimated with the reference approach were 0.174 ± 0.037 mL/min/cm³, 0.342 ± 0.080 1/min, and 0.048 ± 0.009 1/min, respectively. The corresponding parameters obtained with the BS- and image-derived AIFs deviated from these values by 0.6%-18.8% and 16.7%-47.9%, respectively. To compensate for the error in the BS-based technique, data from one MS collected at the end of the experiment were combined with the data from the first 10 min of the BS recording. This approach reduced the deviation in the kinetic parameters to 1.8%-6.3%. Using p/wb led to a 1.7%-8.3% difference from the reference parameters. The sensitivity of the BS was 23%, the energy resolution for the 511-keV photopeak was 19%, and the timing resolution was 11.2 ns. Conclusion: Online recording of the blood activity level with the BS allows precise measurement of AIF, without loss of blood volume. Combining the BS data with one MS is the most accurate approach for the data analysis. The high sensitivity of the device may allow application of lower radioactivity doses.