An optimized registration workflow and standard geometric space for small animal brain imaging.

The reliability of scientific results critically depends on reproducible and transparent data processing. Cross-subject and cross-study comparability of imaging data in general, and magnetic resonance imaging (MRI) data in particular, is contingent on the quality of registration to a standard reference space. In small animal MRI this is not adequately provided by currently used processing workflows, which utilize high-level scripts optimized for human data, and adapt animal data to fit the scripts, rather than vice-versa. In this fully reproducible article we showcase a generic workflow optimized for the mouse brain, alongside a standard reference space suited to harmonize data between analysis and operation. We introduce four separate metrics for automated quality control (QC), and a visualization method to aid operator inspection. Benchmarking this workflow against common legacy practices reveals that it performs more consistently, better preserves variance across subjects while minimizing variance across sessions, and improves both volume and smoothness conservation RMSE approximately 2-fold. We propose this open source workflow and the QC metrics as a new standard for small animal MRI registration, ensuring workflow robustness, data comparability, and region assignment validity, all of which are indispensable prerequisites for the comparability of scientific results across experiments and centers.

Psilocybin exerts distinct effects on resting state networks associated with serotonin and dopamine in mice.

Hallucinogenic agents have been proposed as potent antidepressants; this includes the serotonin (5-HT) receptor 2A agonist psilocybin. In human subjects, psilocybin alters functional connectivity (FC) within the default-mode network (DMN), a constellation of inter-connected regions that displays altered FC in depressive disorders. In this study, we investigated the effects of psilocybin on FC across the entire brain with a view to investigate underlying mechanisms. Psilocybin effects were investigated in lightly-anaesthetized mice using resting-state fMRI. Dual-regression analysis identified reduced FC within the ventral striatum in psilocybin- relative to vehicle-treated mice. Refinement of the analysis using spatial references derived from both gene expression maps and viral tracer projection fields revealed two distinct effects of psilocybin: it increased FC between 5-HT-associated networks and cortical areas, including elements of the murine DMN, thalamus, and midbrain; it decreased FC within dopamine (DA)-associated striatal networks. These results suggest that interactions between 5-HT- and DA-regulated neural networks contribute to the neural and therefore psychological effects of psilocybin. Furthermore, they highlight how information on molecular expression patterns and structural connectivity can assist in the interpretation of pharmaco-fMRI findings.

Pericyte hypoxia-inducible factor-1 (HIF-1) drives blood-brain barrier disruption and impacts acute ischemic stroke outcome.

Pericytes play essential roles in blood-brain barrier integrity and their dysfunction is implicated in neurological disorders such as stroke although the underlying mechanisms remain unknown. Hypoxia-inducible factor-1 (HIF-1), a master regulator of injury responses, has divergent roles in different cells especially during stress scenarios. On one hand HIF-1 is neuroprotective but on the other it induces vascular permeability. Since pericytes are critical for barrier stability, we asked if pericyte HIF-1 signaling impacts barrier integrity and injury severity in a mouse model of ischemic stroke. We show that pericyte HIF-1 loss of function (LoF) diminishes ischemic damage and barrier permeability at 3 days reperfusion. HIF-1 deficiency preserved barrier integrity by reducing pericyte death thereby maintaining vessel coverage and junctional protein organization, and suppressing vascular remodeling. Importantly, considerable improvements in sensorimotor function were observed in HIF-1 LoF mice indicating that better vascular functionality post stroke improves outcome. Thus, boosting vascular integrity by inhibiting pericytic HIF-1 activation and/or increasing pericyte survival may be a lucrative option to accelerate recovery after severe brain injury.

Cortical Excitation:Inhibition Imbalance Causes Abnormal Brain Network Dynamics as Observed in Neurodevelopmental Disorders.

Abnormal brain development manifests itself at different spatial scales. However, whether abnormalities at the cellular level can be diagnosed from network activity measured with functional magnetic resonance imaging (fMRI) is largely unknown, yet of high clinical relevance. Here a putative mechanism reported in neurodevelopmental disorders, that is, excitation-to-inhibition ratio (E:I), was chemogenetically increased within cortical microcircuits of the mouse brain and measured via fMRI. Increased E:I caused a significant "reduction" of long-range connectivity, irrespective of whether excitatory neurons were facilitated or inhibitory Parvalbumin (PV) interneurons were suppressed. Training a classifier on fMRI signals, we were able to accurately classify cortical areas exhibiting increased E:I. This classifier was validated in an independent cohort of Fmr1y/- knockout mice, a model for autism with well-documented loss of parvalbumin neurons and chronic alterations of E:I. Our findings demonstrate a promising novel approach towards inferring microcircuit abnormalities from macroscopic fMRI measurements.

Retrieving neuronal orientations using 3D scanning SAXS and comparison with diffusion MRI.

While diffusion MRI (dMRI) is currently the method of choice to non-invasively probe tissue microstructure and study structural connectivity in the brain, its spatial resolution is limited and its results need structural validation. Current ex vivo methods employed to provide 3D fiber orientations have limitations, including tissue-distorting sample preparation, small field of view or inability to quantify 3D fiber orientation distributions. 3D fiber orientation in tissue sections can be obtained from 3D scanning small-angle X-ray scattering (3D sSAXS) by analyzing the anisotropy of scattering signals. Here we adapt the 3D sSAXS method for use in brain tissue, exploiting the high sensitivity of the SAXS signal to the ordered molecular structure of myelin. We extend the characterization of anisotropy from vectors to tensors, employ the Funk-Radon-Transform for converting scattering information to real space fiber orientations, and demonstrate the feasibility of the method in thin sections of mouse brain with minimal sample preparation. We obtain a second rank tensor representing the fiber orientation distribution function (fODF) for every voxel, thereby generating fODF maps. Finally, we illustrate the potential of 3D sSAXS by comparing the result with diffusion MRI fiber orientations in the same mouse brain. We show a remarkably good correspondence, considering the orthogonality of the two methods, i.e. the different physical processes underlying the two signals. 3D sSAXS can serve as validation method for microstructural MRI, and can provide novel microstructural insights for the nervous system, given the method's orthogonality to dMRI, high sensitivity to myelin sheath's orientation and abundance, and the possibility to extract myelin-specific signal and to perform micrometer-resolution scanning.

Common functional networks in the mouse brain revealed by multi-centre resting-state fMRI analysis.

Preclinical applications of resting-state functional magnetic resonance imaging (rsfMRI) offer the possibility to non-invasively probe whole-brain network dynamics and to investigate the determinants of altered network signatures observed in human studies. Mouse rsfMRI has been increasingly adopted by numerous laboratories worldwide. Here we describe a multi-centre comparison of 17 mouse rsfMRI datasets via a common image processing and analysis pipeline. Despite prominent cross-laboratory differences in equipment and imaging procedures, we report the reproducible identification of several large-scale resting-state networks (RSN), including a mouse default-mode network, in the majority of datasets. A combination of factors was associated with enhanced reproducibility in functional connectivity parameter estimation, including animal handling procedures and equipment performance. RSN spatial specificity was enhanced in datasets acquired at higher field strength, with cryoprobes, in ventilated animals, and under medetomidine-isoflurane combination sedation. Our work describes a set of representative RSNs in the mouse brain and highlights key experimental parameters that can critically guide the design and analysis of future rodent rsfMRI investigations.

Multimodal imaging combining time-domain near-infrared optical tomography and continuous-wave fluorescence molecular tomography.

Fluorescence molecular tomography (FMT) emerges as a powerful non-invasive imaging tool with the ability to resolve fluorescence signals from sources located deep in living tissues. Yet, the accuracy of FMT reconstruction depends on the deviation of the assumed optical properties from the actual values. In this work, we improved the accuracy of the initial optical properties required for FMT using a new-generation time-domain (TD) near-infrared optical tomography (NIROT) system, which effectively decouples scattering and absorption coefficients. We proposed a multimodal paradigm combining TD-NIROT and continuous-wave (CW) FMT. Both numerical simulation and experiments were performed on a heterogeneous phantom containing a fluorescent inclusion. The results demonstrate significant improvement in the FMT reconstruction by taking the NIROT-derived optical properties as prior information. The multimodal method is attractive for preclinical studies and tumor diagnostics since both functional and molecular information can be obtained.

Validation and Comparison of Monte Carlo and Finite Element Method in Forward Modeling for Near Infrared Optical Tomography.

Near infrared optical tomography (NIROT) is a non-invasive imaging technique to provide physiological information e.g. the oxygenation of tissue. For image reconstruction in clinical and preclinical scenarios, models to accurately describe light propagation are needed. This work aims to assess the accuracy and efficiency of different models, which paves the way for an optimal design of model-based image reconstruction algorithms in NIROT for realistic tissue geometries and heterogeneities. Two popular simulators were evaluated: the Monte Carlo (MC) method based MCX and the finite element method (FEM) based Toast++. We compared simulated results with experimental data measured on a homogeneous silicone phantom with well-calibrated parameters. The laser light was focused on the center of the phantom surface and images were captured by a CCD camera in both reflection and transmission modes. For transmittance measurements, the two models showed good agreement. Both achieve a cosine similarity of ~99%. In contrast, for reflectance measurements, FEM results deviated more from the measured values than MC, yielding similarity values of 86% and 94%, respectively. This study recommends the use of MC for NIROT in reflection mode and both MC and FEM yield excellent results for transmission mode.

Inhibiting mGluR5 activity by AFQ056/Mavoglurant rescues circuit-specific functional connectivity in Fmr1 knockout mice.

Previous work has demonstrated that neuroimaging biomarkers which capture functional connectivity of the brain can be used to define a specific and robust endophenotype in Fmr1-/y mice, a well-established animal model of human Fragile-X Syndrome (FXS). However, it is currently unknown whether this macroscopic measure of brain connectivity is sufficiently sensitive to reliably detect changes caused by pharmacological interventions. Here we inhibited the activity of the metabotropic glutamate receptor-5 (mGluR5) using AFQ056/Mavoglurant, a drug that is assumed to normalize excitatory/inhibitory neural signaling imbalances in FXS. We employed resting-state-fMRI (rs-fMRI) and diffusion-weighted imaging (DWI) to test whether Mavoglurant re-established brain connectivity - at least partly - within some of the affected circuits in Fmr1-/y mice that are related to social behavior deficits. In line with previous findings, we observed that Fmr1-/y mice exhibited impaired social interaction, reduced connectivity in three main functional networks and altered network topology. At the group level, Mavoglurant did neither rescue abnormal social behavioral nor white matter abnormalities; however, for some, but not all of these circuits Mavoglurant had a genotype-specific effect of restoring functional connectivity. These results show that rs-fMRI connectivity is sufficiently sensitive to pick up system-level changes after the pharmacological inhibition of mGluR5 activity. However, our results also show that the effects of Mavoglurant are confined to specific networks suggesting that behavioral benefits might be restricted to narrow functional domains.

Dysfunctional Autism Risk Genes Cause Circuit-Specific Connectivity Deficits With Distinct Developmental Trajectories.

Autism spectrum disorders (ASD) are a set of complex neurodevelopmental disorders for which there is currently no targeted therapeutic approach. It is thought that alterations of genes regulating migration and synapse formation during development affect neural circuit formation and result in aberrant connectivity within distinct circuits that underlie abnormal behaviors. However, it is unknown whether deviant developmental trajectories are circuit-specific for a given autism risk-gene. We used MRI to probe changes in functional and structural connectivity from childhood to adulthood in Fragile-X (Fmr1-/y) and contactin-associated (CNTNAP2-/-) knockout mice. Young Fmr1-/y mice (30 days postnatal) presented with a robust hypoconnectivity phenotype in corticocortico and corticostriatal circuits in areas associated with sensory information processing, which was maintained until adulthood. Conversely, only small differences in hippocampal and striatal areas were present during early postnatal development in CNTNAP2-/- mice, while major connectivity deficits in prefrontal and limbic pathways developed between adolescence and adulthood. These findings are supported by viral tracing and electron micrograph approaches and define 2 clearly distinct connectivity endophenotypes within the autism spectrum. We conclude that the genetic background of ASD strongly influences which circuits are most affected, the nature of the phenotype, and the developmental time course of the associated changes.

Structural Basis of Large-Scale Functional Connectivity in the Mouse.

Translational neuroimaging requires approaches and techniques that can bridge between multiple different species and disease states. One candidate method that offers insights into the brain's functional connectivity (FC) is resting-state fMRI (rs-fMRI). In both humans and nonhuman primates, patterns of FC (often referred to as the functional connectome) have been related to the underlying structural connectivity (SC; also called the structural connectome). Given the recent rise in preclinical neuroimaging of mouse models, it is an important question whether the mouse functional connectome conforms to the underlying SC. Here, we compared FC derived from rs-fMRI in female mice with the underlying monosynaptic structural connectome as provided by the Allen Brain Connectivity Atlas. We show that FC between interhemispheric homotopic cortical and hippocampal areas, as well as in cortico-striatal pathways, emerges primarily via monosynaptic structural connections. In particular, we demonstrate that the striatum (STR) can be segregated according to differential rs-fMRI connectivity patterns that mirror monosynaptic connectivity with isocortex. In contrast, for certain subcortical networks, FC emerges along polysynaptic pathways as shown for left and right STR, which do not share direct anatomical connections, but high FC is putatively driven by a top-down cortical control. Finally, we show that FC involving cortico-thalamic pathways is limited, possibly confounded by the effect of anesthesia, small regional size, and tracer injection volume. These findings provide a critical foundation for using rs-fMRI connectivity as a translational tool to study complex brain circuitry interactions and their pathology due to neurological or psychiatric diseases across species.SIGNIFICANCE STATEMENT A comprehensive understanding of how the anatomical architecture of the brain, often referred to as the "connectome," corresponds to its function is arguably one of the biggest challenges for understanding the brain and its pathologies. Here, we use the mouse as a model for comparing functional connectivity (FC) derived from resting-state fMRI with gold standard structural connectivity measures based on tracer injections. In particular, we demonstrate high correspondence between FC measurements of cortico-cortical and cortico-striatal regions and their anatomical underpinnings. This work provides a critical foundation for studying the pathology of these circuits across mouse models and human patients.

Exploring experimental autoimmune optic neuritis using multimodal imaging.

BACKGROUND: Neuro-axonal injury is a key contributor to non-reversible long-term disability in multiple sclerosis (MS). However, the underlying mechanisms are not yet fully understood. Visual impairment is common among MS patients, in which episodes of optic neuritis (ON) are often followed by structural retinal damage and sustained functional impairment. Alterations in the optic nerve and retina have also been described in experimental autoimmune encephalomyelitis (EAE), a rodent model of MS. Thus, investigating structural anterior visual pathway damage may constitute a unique model for assessing mechanisms and temporal sequence of neurodegeneration in MS. We used a multimodal imaging approach utilizing optical coherence tomography (OCT) and diffusion tensor imaging (DTI) to explore the mechanisms and temporal dynamics of visual pathway damage in the animal model of MS. METHODS: 7 EAE-MOG35-55 and 5 healthy female C57BL/6J mice were used in this study. Ganglion cell complex (GCC) thickness was derived from an OCT volume scan centred over the optic nerve head, while the structure of the optic nerve and tracts was assessed from DTI and co-registered T2-weighted sequences performed on a 7T MRI scanner. Data was acquired at baseline, disease onset, peak of disease and recovery. Linear mixed effect models were used to account for intra-subject, inter-eye dependencies, group and time point. Correlation analyses assessed the relationship between GCC thickness and DTI parameters. Immunofluorescence staining of retina and optic nerve sections was used to assess distribution of marker proteins for microglia and neurodegeneration (nerve filaments). RESULTS: In EAE mice, a significant increase in GCC thickness was observed at disease onset (p < 0.001) followed by a decrease at recovery (p < 0.001) compared to controls. The EAE group had significant GCC thinning at recovery compared to all other time points (p < 0.001 for each). Signal increase on T2-weighted images around the optic nerves indicative of inflammation was seen in most of the EAE mice but in none of the controls. A significant decrease in axial diffusivity (AD) and increase in radial diffusivity (RD) values in EAE optic nerves (AD: p = 0.02, RD: p = 0.01) and tract (AD: p = 0.02, RD: p = 0.006) was observed compared to controls. GCC at recovery was positively correlated with AD (optic nerve: rho = 0.74, p = 0.04, optic tract: rho = 0.74, p = 0.04) and negatively correlated with RD (optic nerve: rho = -0.80, p = 0.02, optic tract: rho = -0.75, p = 0.04). Immunofluorescence analysis indicated the presence of activated microglia in the retina and optic nerves in addition to astrocytosis and axonal degeneration in the optic nerve of EAE mice. CONCLUSION: OCT detected GCC changes in EAE may resemble what is observed in MS-related acute ON: an initial phase of swelling (indicative of inflammatory edema) followed by a decrease in thickness over time (representative of neuro-axonal degeneration). In line with OCT findings, DTI of the visual pathway identifies EAE induced pathology (decreased AD, and increased RD). Immunofluorescence analysis provides support for inflammatory pathology and axonal degeneration. OCT together with DTI can detect retinal and optic nerve damage and elucidate to the temporal sequence of neurodegeneration in this rodent model of MS in vivo.

Imaging of cortical structures and microvasculature using extended-focus optical coherence tomography at 1.3 µm.

Extended-focus optical coherence tomography (xf-OCT) is a variant of optical coherence tomography (OCT) wherein the illumination and/or detection modes are engineered to provide a constant diffractionless lateral resolution over an extended depth of field (typically 3 to 10× the Rayleigh range). xf-OCT systems operating at 800 nm have been devised and used in the past to image brain structures at high-resolution in vivo, but are limited to ∼500 µm in penetration depth due to their short illumination wavelength. Here we present an xf-OCT system optimized to an image deeper within the cortex by using a longer illumination central wavelength of 1310 nm. The system offers a lateral resolution of 3 and 6.5 µm, over a depth of 900 µm and >1.5 mm using a 10× and 5× objective, respectively, in air. We characterize the system's resolution using microbeads embedded in PDMS and demonstrate its capabilities by imaging the cortical structure and microvasculature in anesthetized mice to a depth of ∼0.8 mm. Finally, we illustrate the difference in penetration depths obtainable with the new system and an xf-OCT system operating at 800 nm.

Altered regional connectivity reflecting effects of different anaesthesia protocols in the mouse brain.

Studies in mice using resting-state functional magnetic resonance imaging (rs-fMRI) have provided opportunities to investigate the effects of pharmacological manipulations on brain function and map the phenotypes of mouse models of human brain disorders. Mouse rs-fMRI is typically performed under anaesthesia, which induces both regional suppression of brain activity and disruption of large-scale neural networks. Previous comparative studies using rodents investigating various drug effects on long-distance functional connectivity (FC) have reported agent-specific FC patterns, however, effects of regional suppression are sparsely explored. Here we examined changes in regional connectivity under six different anaesthesia conditions using mouse rs-fMRI with the goal of refining the framework of understanding the brain activation under anaesthesia at a local level. Regional homogeneity (ReHo) was used to map local synchronization in the brain, followed by analysis of several brain areas based on ReHo maps. The results revealed high local coherence in most brain areas. The primary somatosensory cortex and caudate-putamen showed agent-specific properties. Lower local coherence in the cingulate cortex was observed under medetomidine, particularly when compared to the combination of medetomidine and isoflurane. The thalamus was associated with retained local coherence across anaesthetic levels and multiple nuclei. These results show that anaesthesia induced by the investigated anaesthetics through different molecular targets promote agent-specific regional connectivity. In addition, ReHo is a data-driven method with minimum user interaction, easy to use and fast to compute. Given that examination of the brain at a local level is widely applied in human rs-fMRI studies, our results show its sensitivity to extract information on varied neuronal activity under six different regimens relevant to mouse functional imaging. These results, therefore, will inform future rs-fMRI studies on mice and the type of anaesthetic agent used, and will help to bridge observations between this burgeoning research field and ongoing human research across analytical scales.

Dynamic reorganization of intrinsic functional networks in the mouse brain.

Functional connectivity (FC) derived from resting-state functional magnetic resonance imaging (rs-fMRI) allows for the integrative study of neuronal processes at a macroscopic level. The majority of studies to date have assumed stationary interactions between brain regions, without considering the dynamic aspects of network organization. Only recently has the latter received increased attention, predominantly in human studies. Applying dynamic FC (dFC) analysis to mice is attractive given the relative simplicity of the mouse brain and the possibility to explore mechanisms underlying network dynamics using pharmacological, environmental or genetic interventions. Therefore, we have evaluated the feasibility and research potential of mouse dFC using the interventions of social stress or anesthesia duration as two case-study examples. By combining a sliding-window correlation approach with dictionary learning, several dynamic functional states (dFS) with a complex organization were identified, exhibiting highly dynamic inter- and intra-modular interactions. Each dFS displayed a high degree of reproducibility upon changes in analytical parameters and across datasets. They fluctuated at different degrees as a function of anesthetic depth, and were sensitive indicators of pathology as shown for the chronic psychosocial stress mouse model of depression. Dynamic functional states are proposed to make a major contribution to information integration and processing in the healthy and diseased brain.

Quantification of antiangiogenic treatment effects on tissue heterogeneity in glioma tumour xenograft model using a combination of DCE-MRI and 3D-ultramicroscopy.

OBJECTIVES: This study aimed at assessing the effects of an anti-angiogenic treatment, which neutralises vascular endothelial growth factor (VEGF), on tumour heterogeneity. METHODS: Murine glioma cells have been inoculated into the right brain frontal lobe of 16 mice. Anti-VEGF antibody was administered to a first group (n = 8), while a second group (n = 8) received a placebo. Magnetic resonance acquisitions, performed at days 10, 12, 15 and 23 following the implantation, allowed the derivation of a three-dimensional features dataset characterising tumour heterogeneity. Three-dimensional ultramicroscopy and standard histochemistry analysis have been performed to verify in vivo results. RESULTS: Placebo-treated mice displayed a highly-vascularised area at the tumour periphery, a monolithic necrotic core and a chaotic dense vasculature across the entire tumour. In contrast, the B20-treated group did not show any highly vascularised regions and presents a fragmented necrotic core. A significant reduction of the number of vessel segments smaller than 17 µm has been observed. There was no difference in overall tumour volume and growth rate between the two groups. CONCLUSIONS: Region-specific analysis revealed that VEGF inhibition affects only: (1) highly angiogenic compartments expressing high levels of VEGF and characterised by small capillaries, and also (2) the formation and structure of necrotic regions. These effects appear to be transient and limited in time. KEY POINTS: • VEGF inhibition affects only the highly angiogenic region and small capillaries network • VEGF inhibition is transient in time • Tumour volume is not affected by anti-angiogenic treatment • VEGF inhibition also influences the architecture of necrotic regions.

Complex interplay between brain function and structure during cerebral amyloidosis in APP transgenic mouse strains revealed by multi-parametric MRI comparison.

Alzheimer's disease is a fatal neurodegenerative disorder affecting the aging population. Neuroimaging methods, in particular magnetic resonance imaging (MRI), have helped reveal alterations in the brain structure, metabolism, and function of patients and in groups at risk of developing AD, yet the nature of these alterations is poorly understood. Neuroimaging in mice is attractive for investigating mechanisms underlying functional and structural changes associated with AD pathology. Several preclinical murine models of AD have been generated based on transgenic insertion of human mutated APP genes. Depending on the specific mutations, mouse strains express different aspects of amyloid pathology, e.g. intracellular amyloid-ß (Aß) aggregates, parenchymal plaques, or cerebral amyloid angiopathy. We have applied multi-parametric MRI in three transgenic mouse lines to compare changes in brain function with resting-state fMRI and structure with diffusion tensor imaging and high resolution anatomical imaging. E22ΔAß developing intracellular Aß aggregates did not present functional or structural alterations compared to their wild-type littermates. PSAPP mice displaying parenchymal amyloid plaques displayed mild functional changes within the supplementary and barrel field cortices, and increased isocortical volume relative to controls. Extensive reduction in functional connectivity in the sensory-motor cortices and within the default mode network, as well as local volume increase in the midbrain relative to wild-type have been observed in ArcAß mice bearing intracellular Aß aggregates as well as parenchymal and vascular amyloid deposits. Patterns of functional and structural changes appear to be strain-specific and not directly related to amyloid deposition.

Chronic psychosocial stress in mice leads to changes in brain functional connectivity and metabolite levels comparable to human depression.

Human depression, for which chronic psychosocial stress is a major risk factor, is characterized by consistent alterations in neurocircuitry. For example, there is increased functional connectivity (FC) within and between regions comprising the default mode network (DMN) including prefrontal cortex and cingulate cortex. Alterations in network FC are associated with specific aspects of psychopathology. In mice, chronic psychosocial stress (CPS) leads to depression-relevant behavior, including increased fear learning, learned helplessness, fatigue and decreased motivation for reward. Using multimodal in vivo magnetic resonance imaging (MRI) and spectroscopy (MRS), we investigated CPS effects on function and structure in the mouse brain under light anesthesia. Mice underwent a baseline MRI/MRS session, followed by 15-day CPS (n=26) or control handling (n=27), and a post-treatment MRI/MRS session. In BOLD fMRI, relative to controls, CPS mice exhibited robust, reproducible increases in FC within 8 of 9 identified cortical networks, including the prefrontal and cingulate cortices that contribute to the "mouse DMN". CPS mice exhibited increases in between-network FC, including amygdala - prefrontal cortex and amygdala - cingulate cortex. MRS identified metabolic alterations in CPS mice as increased inositol levels in amygdala and increased glycerophosphorylcholine levels in prefrontal cortex. Diffusion-weighted MRI detected increased fractional anisotropic values in the cingulum. This study demonstrates that chronic psychosocial stress induces FC states in the mouse brain analogous to those observed in depression, as well as cerebral metabolism and white matter pathway alterations that contribute to understanding of pathological processes. It also demonstrates the importance of brain imaging to the establishment of valid animal models in translational psychiatry.

Genetic enhancement of microsomal epoxide hydrolase improves metabolic detoxification but impairs cerebral blood flow regulation.

Microsomal epoxide hydrolase (mEH) is a detoxifying enzyme for xenobiotic compounds. Enzymatic activity of mEH can be greatly increased by a point mutation, leading to an E404D amino acid exchange in its catalytic triad. Surprisingly, this variant is not found in any vertebrate species, despite the obvious advantage of accelerated detoxification. We hypothesized that this evolutionary avoidance is due to the fact that the mEH plays a dualistic role in detoxification and control of endogenous vascular signaling molecules. To test this, we generated mEH E404D mice and assessed them for detoxification capacity and vascular dynamics. In liver microsomes from these mice, turnover of the xenobiotic compound phenanthrene-9,10-oxide was four times faster compared to WT liver microsomes, confirming accelerated detoxification. mEH E404D animals also showed faster metabolization of a specific class of endogenous eicosanoids, arachidonic acid-derived epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs). Significantly higher DHETs/EETs ratios were found in mEH E404D liver, urine, plasma, brain and cerebral endothelial cells compared to WT controls, suggesting a broad impact of the mEH mutant on endogenous EETs metabolism. Because EETs are strong vasodilators in cerebral vasculature, hemodynamics were assessed in mEH E404D and WT cerebral cortex and hippocampus using cerebral blood volume (CBV)-based functional magnetic resonance imaging (fMRI). Basal CBV0 levels were similar between mEH E404D and control mice in both brain areas. But vascular reactivity and vasodilation in response to the vasodilatory drug acetazolamide were reduced in mEH E404D forebrain compared to WT controls by factor 3 and 2.6, respectively. These results demonstrate a critical role for mEH E404D in vasodynamics and suggest that deregulation of endogenous signaling pathways is the undesirable gain of function associated with the E404D variant.

Early alterations in functional connectivity and white matter structure in a transgenic mouse model of cerebral amyloidosis.

Impairment of brain functional connectivity (FC) is thought to be an early event occurring in diseases with cerebral amyloidosis, such as Alzheimer's disease. Regions sustaining altered functional networks have been shown to colocalize with regions marked with amyloid plaques burden suggesting a strong link between FC and amyloidosis. Whether the decline in FC precedes amyloid plaque deposition or is a consequence thereof is currently unknown. The sequence of events during early stages of the disease is difficult to capture in humans due to the difficulties in providing an early diagnosis and also in view of the heterogeneity among patients. Transgenic mouse lines overexpressing amyloid precursor proteins develop cerebral amyloidosis and constitute an attractive model system for studying the relationship between plaque and functional changes. In this study, ArcAß transgenic and wild-type mice were imaged using resting-state fMRI methods across their life-span in a cross-sectional design to analyze changes in FC in relation to the pathology. Transgenic mice show compromised development of FC during the first months of postnatal life compared with wild-type animals, resulting in functional impairments that affect in particular the sensory-motor cortex already in preplaque stage. These functional alterations were accompanied by structural changes as reflected by reduced fractional anisotropy values, as derived from diffusion tensor imaging. Our results suggest cerebral amyloidosis in mice is preceded by impairment of neuronal networks and white matter structures. FC analysis in mice is an attractive tool for studying the implications of impaired neuronal networks in models of cerebral amyloid pathology.