Virtual histology of Alzheimer's disease: Biometal entrapment within amyloid-ß plaques allows for detection via X-ray phase-contrast imaging.

Amyloid-ß (Aß) plaques from Alzheimer's Disease (AD) can be visualized ex vivo in label-free brain samples using synchrotron X-ray phase-contrast tomography (XPCT). However, for XPCT to be useful as a screening method for amyloid pathology, it is essential to understand which factors drive the detection of Aß plaques. The current study was designed to test the hypothesis that Aß-related contrast in XPCT could be caused by Aß fibrils and/or by metals trapped in the plaques. Fibrillar and elemental compositions of Aß plaques were probed in brain samples from different types of AD patients and AD models to establish a relationship between XPCT contrast and Aß plaque characteristics. XPCT, micro-Fourier-Transform Infrared spectroscopy and micro-X-Ray Fluorescence spectroscopy were conducted on human samples (one genetic and one sporadic case) and on four transgenic rodent strains (mouse: APPPS1, ArcAß, J20; rat: TgF344). Aß plaques from the genetic AD patient were visible using XPCT, and had higher ß-sheet content and higher metal levels than those from the sporadic AD patient, which remained undetected by XPCT. Aß plaques in J20 mice and TgF344 rats appeared hyperdense on XPCT images, while they were hypodense with a hyperdense core in the case of APPPS1 and ArcAß mice. In all four transgenic strains, ß-sheet content was similar, while metal levels were highly variable: J20 (zinc and iron) and TgF344 (copper) strains showed greater metal accumulation than APPPS1 and ArcAß mice. Hence, a hyperdense contrast formation of Aß plaques in XPCT images was associated with biometal entrapment within plaques. STATEMENT OF SIGNIFICANCE: The role of metals in Alzheimer's disease (AD) has been a subject of continuous interest. It was already known that amyloid-ß plaques (Aß), the earliest hallmark of AD, tend to trap endogenous biometals like zinc, iron and copper. Here we show that this metal accumulation is the main reason why Aß plaques are detected with a new technique called X-ray phase contrast tomography (XPCT). XPCT enables to map the distribution of Aß plaques in the whole excised brain without labeling. In this work we describe a unique collection of four transgenic models of AD, together with a human sporadic and a rare genetic case of AD, thus exploring the full spectrum of amyloid contrast in XPCT.

Revealing the three-dimensional murine brain microstructure by contrast-enhanced computed tomography.

To improve our understanding of the brain microstructure, high-resolution 3D imaging is used to complement classical 2D histological assessment techniques. X-ray computed tomography allows high-resolution 3D imaging, but requires methods for enhancing contrast of soft tissues. Applying contrast-enhancing staining agents (CESAs) ameliorates the X-ray attenuating properties of soft tissue constituents and is referred to as contrast-enhanced computed tomography (CECT). Despite the large number of chemical compounds that have successfully been applied as CESAs for imaging brain, they are often toxic for the researcher, destructive for the tissue and without proper characterization of affinity mechanisms. We evaluated two sets of chemically related CESAs (organic, iodinated: Hexabrix and CA4+ and inorganic polyoxometalates: 1:2 hafnium-substituted Wells-Dawson phosphotungstate and Preyssler anion), for CECT imaging of healthy murine hemispheres. We then selected the CESA (Hexabrix) that provided the highest contrast between gray and white matter and applied it to a cuprizone-induced demyelination model. Differences in the penetration rate, effect on tissue integrity and affinity for tissue constituents have been observed for the evaluated CESAs. Cuprizone-induced demyelination could be visualized and quantified after Hexabrix staining. Four new non-toxic and non-destructive CESAs to the field of brain CECT imaging were introduced. The added value of CECT was shown by successfully applying it to a cuprizone-induced demyelination model. This research will prove to be crucial for further development of CESAs for ex vivo brain CECT and 3D histopathology.

Head down tilt 15° to preserve salvageable brain tissue in acute ischemic stroke: A pre-clinical pooled analysis, with focus on cerebral hemodynamics.

Neurological outcome after ischemic stroke depends on residual salvageable brain tissue at the time of recanalization. Head down tilt 15° (HDT15) was proven effective in reducing infarct size and improving functional outcome in rats with transient middle cerebral artery occlusion (t-MCAO) by increasing cerebral perfusion within the ischemic penumbra. In this pooled analysis, individual animal-level data from three experimental series were combined in a study population of 104 t-MCAO rats (45 in HDT15 group and 59 in flat position group). Co-primary outcomes were infarct size and functional outcome at 24 h in both groups. The secondary outcome was hemodynamic change induced by HDT15 in ischemic and non-ischemic hemispheres in a subgroup of animals. Infarct size at 24 h was smaller in HDT15 group than in flat position group (absolute mean difference 31.69 mm3 , 95% CI 9.1-54.2, Cohen's d 0.56, p = 0.006). Functional outcome at 24 h was better in HDT15 group than in flat position group (median [IQR]: 13[10-16] vs. 11), with a shift in the distribution of the neurobehavioural scores in favour of HDT15. Mean cerebral perfusion in the ischemic hemisphere was higher during HDT15 than before its application (Perfusion Unit [P.U.], mean ± SD: 52.5 ± 19.52 P.U. vs. 41.25 ± 14.54 P.U., mean of differences 13.36, 95% CI 7.5-19.18, p = 0.0002). Mean cerebral perfusion in the non-ischemic hemisphere before and during HDT15 was unchanged (P.U., mean ± SD: 94.1 ± 33.8 P.U. vs. 100.25 ± 25.34 P.U., mean of differences 3.95, 95%, CI -1.9 to 9.6, p = 0.1576). This study confirmed that HDT15 improves the outcome in t-MCAO rats by promoting cerebral perfusion in the ischemic territory, without disrupting hemodynamics in non-ischemic areas.

Use of metal-based contrast agents for in vivo MR and CT imaging of phagocytic cells in neurological pathologies.

The activation of phagocytic cells is a hallmark of many neurological diseases. Imaging them in their 3-dimensional cerebral environment over time is crucial to better understand their role in disease pathogenesis and to monitor their potential therapeutic effects. Phagocytic cells have the ability to internalize metal-based contrast agents both in vitro and in vivo and can thus be tracked by magnetic resonance imaging (MRI) or computed tomography (CT). In this review article, we summarize the different labelling strategies, contrast agents, and in vivo imaging modalities that can be used to monitor cells with phagocytic activity in the central nervous system using MRI and CT, with a focus on clinical applications. Metal-based nanoparticle contrast agents such as gadolinium, gold and iron are ideal candidates for these applications as they have favourable magnetic and/or radiopaque properties and can be fine-tuned for optimal uptake by phagocytic cells. However, they also come with downsides due to their potential toxicity, especially in the brain where they might accumulate. We therefore conclude our review by discussing the pitfalls, safety and potential for clinical translation of these metal-based neuroimaging techniques. Early results in patients with neuropathologies such as multiple sclerosis, stroke, trauma, cerebral aneurysm and glioblastoma are promising. If the challenges represented by safety issues are overcome, phagocytic cells imaging will be a very valuable tool for studying and understanding the inflammatory response and evaluating treatments that aim at mitigating this response in patients with neurological diseases.

Charge detection mass spectrometry on human-amplified fibrils from different synucleinopathies.

Amyloid fibrils are self-assembled mesoscopic protein aggregates, which can accumulate to form deposits or plaques in the brain. In vitro amplification of fibrils can be achieved with real-time quaking-induced conversion (RT-QuIC). However, this emerging technique would benefit from a complementary method to assess structural properties of the amplification products. This work demonstrates the feasibility of nanospray-charge-detection-mass-spectrometry (CDMS) performed on α-synuclein (αSyn) fibrils amplified from human brains with Parkinson's disease (PD) or Dementia with Lewy bodies (DLB) and its synergistic combination with RT-QuIC.

Brain virtual histology with X-ray phase-contrast tomography Part II:3D morphologies of amyloid-ß plaques in Alzheimer's disease models.

While numerous transgenic mouse strains have been produced to model the formation of amyloid-ß (Aß) plaques in the brain, efficient methods for whole-brain 3D analysis of Aß deposits have to be validated and standardized. Moreover, routine immunohistochemistry performed on brain slices precludes any shape analysis of Aß plaques, or require complex procedures for serial acquisition and reconstruction. The present study shows how in-line (propagation-based) X-ray phase-contrast tomography (XPCT) combined with ethanol-induced brain sample dehydration enables hippocampus-wide detection and morphometric analysis of Aß plaques. Performed in three distinct Alzheimer mouse strains, the proposed workflow identified differences in signal intensity and 3D shape parameters: 3xTg displayed a different type of Aß plaques, with a larger volume and area, greater elongation, flatness and mean breadth, and more intense average signal than J20 and APP/PS1. As a label-free non-destructive technique, XPCT can be combined with standard immunohistochemistry. XPCT virtual histology could thus become instrumental in quantifying the 3D spreading and the morphological impact of seeding when studying prion-like properties of Aß aggregates in animal models of Alzheimer's disease. This is Part II of a series of two articles reporting the value of in-line XPCT for virtual histology of the brain; Part I shows how in-line XPCT enables 3D myelin mapping in the whole rodent brain and in human autopsy brain tissue.

Brain virtual histology with X-ray phase-contrast tomography Part I: whole-brain myelin mapping in white-matter injury models.

White-matter injury leads to severe functional loss in many neurological diseases. Myelin staining on histological samples is the most common technique to investigate white-matter fibers. However, tissue processing and sectioning may affect the reliability of 3D volumetric assessments. The purpose of this study was to propose an approach that enables myelin fibers to be mapped in the whole rodent brain with microscopic resolution and without the need for strenuous staining. With this aim, we coupled in-line (propagation-based) X-ray phase-contrast tomography (XPCT) to ethanol-induced brain sample dehydration. We here provide the proof-of-concept that this approach enhances myelinated axons in rodent and human brain tissue. In addition, we demonstrated that white-matter injuries could be detected and quantified with this approach, using three animal models: ischemic stroke, premature birth and multiple sclerosis. Furthermore, in analogy to diffusion tensor imaging (DTI), we retrieved fiber directions and DTI-like diffusion metrics from our XPCT data to quantitatively characterize white-matter microstructure. Finally, we showed that this non-destructive approach was compatible with subsequent complementary brain sample analysis by conventional histology. In-line XPCT might thus become a novel gold-standard for investigating white-matter injury in the intact brain. This is Part I of a series of two articles reporting the value of in-line XPCT for virtual histology of the brain; Part II shows how in-line XPCT enables the whole-brain 3D morphometric analysis of amyloid- ß (A ß ) plaques.

Neurofunctional and neuroimaging readouts for designing a preclinical stem-cell therapy trial in experimental stroke.

With the aim of designing a preclinical study evaluating an intracerebral cell-based therapy for stroke, an observational study was performed in the rat suture model of ischemic stroke. Objectives were threefold: (i) to characterize neurofunctional and imaging readouts in the first weeks following transient ischemic stroke, according to lesion subtype (hypothalamic, striatal, corticostriatal); (ii) to confirm that intracerebral administration does not negatively impact these readouts; and (iii) to calculate sample sizes for a future therapeutic trial using these readouts as endpoints. Our results suggested that the most relevant endpoints were side bias (staircase test) and axial diffusivity (AD) (diffusion tensor imaging). Hypothalamic-only lesions did not affect those parameters, which were close to normal. Side bias in striatal lesions reached near-normal levels within 2 weeks, while rats with corticostriatal lesions remained impaired until week 14. AD values were decreased at 4 days and increased at 5 weeks post-surgery, with a subtype gradient: hypothalamic < striatal < corticostriatal. Intracerebral administration did not impact these readouts. After sample size calculation (18-147 rats per group according to the endpoint considered), we conclude that a therapeutic trial based on both readouts would be feasible only in the framework of a multicenter trial.

In vivo targeting and multimodal imaging of cerebral amyloid-ß aggregates using hybrid GdF3 nanoparticles.

Aim: To propose a new multimodal imaging agent targeting amyloid-ß (Aß) plaques in Alzheimer's disease. Materials & methods: A new generation of hybrid contrast agents, based on gadolinium fluoride nanoparticles grafted with a pentameric luminescent-conjugated polythiophene, was designed, extensively characterized and evaluated in animal models of Alzheimer's disease through MRI, two-photon microscopy and synchrotron x-ray phase-contrast imaging. Results & conclusion: Two different grafting densities of luminescent-conjugated polythiophene were achieved while preserving colloidal stability and fluorescent properties, and without affecting biodistribution. In vivo brain uptake was dependent on the blood-brain barrier status. Nevertheless, multimodal imaging showed successful Aß targeting in both transgenic mice and Aß fibril-injected rats.

The design and study of a new contrast agent targeting amyloid-ß (Aß) plaques in Alzheimer's disease (AD) is proposed. Aß plaques are the earliest pathological sign of AD, silently appearing in the brain decades before the symptoms of the disease are manifested. While current detection of Aß plaques is based on nuclear medicine (a technique using a radioactive agent), a different kind of contrast agent is here evaluated in animal models of AD. The contrast agent consists of a nanoparticle made of gadolinium and fluorine ions (core), and decorated with a molecule previously shown to bind to Aß plaques (grafting). The core is detectable with MRI and x-ray imaging, while the grafting molecule is detectable with fluorescence imaging, thus allowing different imaging methods to be combined to study the pathology. In this work, the structure, stability and properties of the contrast agent have been verified in vitro (in tubes and on brain sections). Then the ability of the contrast agent to bind to Aß plaques and provide a detectable signal in MRI, x-ray or fluorescence imaging has been demonstrated in vivo (in rodent models of AD). This interdisciplinary research establishes the proof of concept that this new class of versatile agent contrast can be used to target pathological processes in the brain.

Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies.

PURPOSE: The prototypical TSPO radiotracer (R)-[11C]PK11195 has been used in humans for more than thirty years to visualize neuroinflammation in several pathologies. Alternative radiotracers have been developed to improve signal-to-noise ratio and started to be tested clinically in 2008. Here we examined the scientific value of these "(R)-[11C]PK11195 challengers" in clinical research to determine if they could supersede (R)-[11C]PK11195. METHODS: A systematic MEDLINE (PubMed) search was performed (up to end of year 2020) to extract publications reporting TSPO PET in patients with identified pathologies, excluding studies in healthy subjects and methodological studies. RESULTS: Of the 288 publications selected, 152 used 13 challengers, and 142 used (R)-[11C]PK11195. Over the last 20 years, the number of (R)-[11C]PK11195 studies remained stable (6 ± 3 per year), but was surpassed by the total number of challenger studies for the last 6 years. In total, 3914 patients underwent a TSPO PET scan, and 47% (1851 patients) received (R)-[11C]PK11195. The 2 main challengers were [11C]PBR28 (24%-938 patients) and [18F]FEPPA (11%-429 patients). Only one-in-ten patients (11%-447) underwent 2 TSPO scans, among whom 40 (1%) were scanned with 2 different TSPO radiotracers. CONCLUSIONS: Generally, challengers confirmed disease-specific initial (R)-[11C]PK11195 findings. However, while their better signal-to-noise ratio seems particularly useful in diseases with moderate and widespread neuroinflammation, most challengers present an allelic-dependent (Ala147Thr polymorphism) TSPO binding and genetic stratification is hindering their clinical implementation. As new challengers, insensitive to TSPO human polymorphism, are about to enter clinical evaluation, we propose this systematic review to be regularly updated (living review).

18F-florbetapir PET/MRI for quantitatively monitoring myelin loss and recovery in patients with multiple sclerosis: A longitudinal study.

BACKGROUND: Amyloid positron emission tomography (PET) can measure in-vivo demyelination in patients with multiple sclerosis (MS). However, the value of 18F-labeled amyloid PET tracer, 18F-florbetapir in the longitudinal study for monitoring myelin loss and recovery has not been confirmed. METHODS: From March 2019 to September 2020, twenty-three patients with MS and nine healthy controls (HCs) underwent a hybrid PET/MRI at baseline and expanded disability status scale (EDSS) assessment, and eight of 23 patients further underwent follow-up PET/MRI. The distribution volume ratio (DVR) and standard uptake value ratio (SUVR) of 18F-florbetapir in damaged white matter (DWM) and normal-appearance white matter (NAWM) were obtained from dynamic and static PET acquisition. Diffusion tensor imaging-derived parameters were also calculated. Data were expressed as mean ± standard deviation with 99% confidence interval (99%CI). FINDING: The mean DVR (1.08 ± 0.12, 99%CI [1.02 ~ 1.14]) but not the mean SUVR of DWM lesions was lower than that of NAWM in patients with MS (1.25 ± 0.10, 99%CI [1.20 ~ 1.31]) and HCs (1.29 ± 0.08, 99%CI [1.23 ~ 1.36]). A trend toward lower mean fractional anisotropy (374.95 ± 45.30 vs. 419.07 ± 4.83) and higher mean radial diffusivity (0.45 ± 0.05 vs. 0.40 ± 0.01) of NAWM in patients with MS than those in HCs was found. DVR decreased in DWM lesions with higher MD (rho = -0.261, 99%CI [-0.362 ~ -0.144]), higher AD (rho = -0.200, 99%CI [-0.318 ~ -0.070]) and higher RD (rho = -0.198, 99%CI [-0.313 ~ -0.075]). Patients' EDSS scores were reduced (B = 0.04, 99%CI [-0.005 ~ 0.084]) with decreased index of global demyelination in the longitudinal study. INTERPRETATION: Our exploratory study suggests that dynamic 18F-florbetapir PET/MRI may be a very promising tool for quantitatively monitoring myelin loss and recovery in patients with MS. FUNDING: Shanghai Pujiang Program, Shanghai Municipal Key Clinical Specialty, Shanghai Shuguang Plan Project, Shanghai Health and Family Planning Commission Research Project, Clinical Research Plan of SHDC, French-Chinese program "Xu Guangqi".

Multimodal Imaging with NanoGd Reveals Spatiotemporal Features of Neuroinflammation after Experimental Stroke.

The purpose of this study is to propose and validate a preclinical in vivo magnetic resonance imaging (MRI) tool to monitor neuroinflammation following ischemic stroke, based on injection of a novel multimodal nanoprobe, NanoGd, specifically designed for internalization by phagocytic cells. First, it is verified that NanoGd is efficiently internalized by microglia in vitro. In vivo MRI coupled with intravenous injection of NanoGd in a permanent middle cerebral artery occlusion mouse model results in hypointense signals in the ischemic lesion. In these mice, longitudinal two-photon intravital microscopy shows NanoGd internalization by activated CX3CR1-GFP/+ cells. Ex vivo analysis, including phase contrast imaging with synchrotron X-ray, histochemistry, and transmission electron microscopy corroborate NanoGd accumulation within the ischemic lesion and uptake by immune phagocytic cells. Taken together, these results confirm the potential of NanoGd-enhanced MRI as an imaging biomarker of neuroinflammation at the subacute stage of ischemic stroke. As far as it is known, this work is the first to decipher the working mechanism of MR signals induced by a nanoparticle passively targeted at phagocytic cells by performing intravital microscopy back-to-back with MRI. Furthermore, using a gadolinium-based rather than an iron-based contrast agent raises future perspectives for the development of molecular imaging with emerging computed tomography technologies.

Neuroprotection by remote ischemic conditioning in the setting of acute ischemic stroke: a preclinical two-centre study.

Reperfusion is the only existing strategy for patients with acute ischemic stroke, however it causes further brain damage itself. A feasible therapy targeting reperfusion injury is remote ischemic conditioning (RIC). This was a two-centre, randomized, blinded international study, using translational imaging endpoints, aimed to examine the neuroprotective effects of RIC in ischemic stroke model. 80 male rats underwent 90-min middle cerebral artery occlusion. RIC consisted of 4 × 5 min cycles of left hind limb ischemia. The primary endpoint was infarct size measured on T2-weighted MRI at 24 h, expressed as percentage of the area-at-risk. Secondary endpoints were: hemispheric space-modifying edema, infarct growth between per-occlusion and 24 h MRI, neurofunctional outcome measured by neuroscores. 47 rats were included in the analysis after applying pre-defined inclusion criteria. RIC significantly reduced infarct size (median, interquartile range: 19% [8%; 32%] vs control: 40% [17%; 59%], p = 0.028). This effect was still significant after adjustment for apparent diffusion coefficient lesion size in multivariate analysis. RIC also improved neuroscores (6 [3; 8] vs control: 9 [7; 11], p = 0.032). Other secondary endpoints were not statistically different between groups. We conclude that RIC in the setting of acute ischemic stroke in rats is safe, reduces infarct size and improves functional recovery.

Change in Expression of 5-HT6 Receptor at Different Stages of Alzheimer's Disease: A Postmortem Study with the PET Radiopharmaceutical [18F]2FNQ1P.

BACKGROUND: The 5-HT6 receptor is one of the most recently identified serotonin receptors in the central nervous system. Because of its role in memory and cognitive process, this receptor might be implicated in Alzheimer's disease (AD) and associated disorders. OBJECTIVE: The aim of this study was to investigate the binding of [18F]2FNQ1P, a new specific radiotracer of 5-HT6 receptors, and to quantify 5-HT6 receptor density in caudate nucleus in a population of patients with different AD stages. METHODS: Patients were classified according to the "ABC" NIA-AA classification. In vitro binding assays were performed in postmortem brain tissue from the healthy control (HC; n = 8) and severe AD ("High"; n = 8) groups. In vitro quantitative autoradiography was performed in human brain tissue (caudate nucleus) from patients with different stages of AD: HC (n = 15), "Low" (n = 18), "Int" (n = 20), and "High" (n = 15). RESULTS: In vitro binding assays did not show significant differences for the KD and Bmax parameters between "High" and HC groups. In vitro quantitative autoradiography showed a significant difference between the "High" and HC groups (p = 0.0025). We also showed a progressive diminution in [18F]2FNQ1P specific binding, which parallels 5-HT6 receptors expression, according to increasing AD stage. Significant differences were observed between the HC group and all AD stages combined ("Low", "Intermediate", and "High") (p = 0.011). CONCLUSION: This study confirms the interest of investigating the role of 5-HT6 receptors in AD and related disorders. [18F]2FNQ1P demonstrated specific binding to 5-HT6 receptors.

The apparent mechanical effect of isolated amyloid-ß and α-synuclein aggregates revealed by multi-frequency MRE.

Several biological processes are involved in dementia, and fibrillar aggregation of misshaped endogenous proteins appears to be an early hallmark of neurodegenerative disease. A recently developed means of studying neurodegenerative diseases is magnetic resonance elastography (MRE), an imaging technique investigating the mechanical properties of tissues. Although mechanical changes associated with these diseases have been detected, the specific signal of fibrils has not yet been isolated in clinical or preclinical studies. The current study aims to exploit the fractal-like properties of fibrils to separate them from nonaggregated proteins using a multi-frequency MRE power law exponent in a phantom study. Two types of fibril, α-synuclein (α-Syn) and amyloid-ß (Aß), and a nonaggregated protein, bovine serum albumin, used as control, were incorporated in a dedicated nondispersive agarose phantom. Elastography was performed at multiple frequencies between 400 and 1200 Hz. After 3D-direct inversion, storage modulus (G'), phase angle (ϕ), wave speed and the power law exponent (y) were computed. No significant changes in G' and ϕ were detected. Both α-Syn and Aß inclusions showed significantly higher y values than control inclusions (P = 0.005) but did not differ between each other. The current phantom study highlighted a specific biomechanical effect of α-Syn and Aß aggregates, which was better captured with the power law exponent derived from multi-frequency MRE than with single frequency-derived parameters.

Evaluation of Myelin Radiotracers in the Lysolecithin Rat Model of Focal Demyelination: Beware of Pitfalls!

The observation that amyloid radiotracers developed for Alzheimer's disease bind to cerebral white matter paved the road to nuclear imaging of myelin in multiple sclerosis. The lysolecithin (lysophosphatidylcholine (LPC)) rat model of demyelination proved useful in evaluating and comparing candidate radiotracers to target myelin. Focal demyelination following stereotaxic LPC injection is larger than lesions observed in experimental autoimmune encephalitis models and is followed by spontaneous progressive remyelination. Moreover, the contralateral hemisphere may serve as an internal control in a given animal. However, demyelination can be accompanied by concurrent focal necrosis and/or adjacent ventricle dilation. The influence of these side effects on imaging findings has never been carefully assessed. The present study describes an optimization of the LPC model and highlights the use of MRI for controlling the variability and pitfalls of the model. The prototypical amyloid radiotracer [11C]PIB was used to show that in vivo PET does not provide sufficient sensitivity to reliably track myelin changes and may be sensitive to LPC side effects instead of demyelination as such. Ex vivo autoradiography with a fluorine radiotracer should be preferred, to adequately evaluate and compare radiotracers for the assessment of myelin content.

MRI coupled with clinically-applicable iron oxide nanoparticles reveals choroid plexus involvement in a murine model of neuroinflammation.

Choroid plexus (ChPs) are involved in the early inflammatory response that occurs in many brain disorders. However, the activation of immune cells within the ChPs in response to neuroinflammation is still largely unexplored in-vivo. There is therefore a crucial need for developing imaging tool that would allow the non-invasive monitoring of ChP involvement in these diseases. Magnetic resonance imaging (MRI) coupled with superparamagnetic particles of iron oxide (SPIO) is a minimally invasive technique allowing to track phagocytic cells in inflammatory diseases. Our aim was to investigate the potential of ultrasmall SPIO (USPIO)-enhanced MRI to monitor ChP involvement in-vivo in a mouse model of neuroinflammation obtained by intraperitoneal administration of lipopolysaccharide. Using high resolution MRI, we identified marked USPIO-related signal drops in the ChPs of animals with neuroinflammation compared to controls. We confirmed these results quantitatively using a 4-points grading system. Ex-vivo analysis confirmed USPIO accumulation within the ChP stroma and their uptake by immune cells. We validated the translational potential of our approach using the clinically-applicable USPIO Ferumoxytol. MR imaging of USPIO accumulation within the ChPs may serve as an imaging biomarker to study ChP involvement in neuroinflammatory disorders that could be applied in a straightforward way in clinical practice.

Clinical Imaging of Choroid Plexus in Health and in Brain Disorders: A Mini-Review.

The choroid plexuses (ChPs) perform indispensable functions for the development, maintenance and functioning of the brain. Although they have gained considerable interest in the last years, their involvement in brain disorders is still largely unknown, notably because their deep location inside the brain hampers non-invasive investigations. Imaging tools have become instrumental to the diagnosis and pathophysiological study of neurological and neuropsychiatric diseases. This review summarizes the knowledge that has been gathered from the clinical imaging of ChPs in health and brain disorders not related to ChP pathologies. Results are discussed in the light of pre-clinical imaging studies. As seen in this review, to date, most clinical imaging studies of ChPs have used disease-free human subjects to demonstrate the value of different imaging biomarkers (ChP size, perfusion/permeability, glucose metabolism, inflammation), sometimes combined with the study of normal aging. Although very few studies have actually tested the value of ChP imaging biomarkers in patients with brain disorders, these pioneer studies identified ChP changes that are promising data for a better understanding and follow-up of diseases such as schizophrenia, epilepsy and Alzheimer's disease. Imaging of immune cell trafficking at the ChPs has remained limited to pre-clinical studies so far but has the potential to be translated in patients for example using MRI coupled with the injection of iron oxide nanoparticles. Future investigations should aim at confirming and extending these findings and at developing translational molecular imaging tools for bridging the gap between basic molecular and cellular neuroscience and clinical research.

Magnetic Resonance Elastography of Rodent Brain.

Magnetic resonance elastography (MRE) is a non-invasive imaging technique, using the propagation of mechanical waves as a probe to palpate biological tissues. It consists in three main steps: production of shear waves within the tissue; encoding subsequent tissue displacement in magnetic resonance images; and extraction of mechanical parameters based on dedicated reconstruction methods. These three steps require an acoustic-frequency mechanical actuator, magnetic resonance imaging acquisition, and a post-processing tool for which no turnkey technology is available. The aim of the present review is to outline the state of the art of reported set-ups to investigate rodent brain mechanical properties. The impact of experimental conditions in dimensioning the set-up (wavelength and amplitude of the propagated wave, spatial resolution, and signal-to-noise ratio of the acquisition) on the accuracy and precision of the extracted parameters is discussed, as well as the influence of different imaging sequences, scanners, electromagnetic coils, and reconstruction algorithms. Finally, the performance of MRE in demonstrating viscoelastic differences between structures constituting the physiological rodent brain, and the changes in brain parameters under pathological conditions, are summarized. The recently established link between biomechanical properties of the brain as obtained on MRE and structural factors assessed by histology is also studied. This review intends to give an accessible outline on how to conduct an elastography experiment, and on the potential of the technique in providing valuable information for neuroscientists.