Centrosome anchoring regulates progenitor properties and cortical formation.

Radial glial progenitor cells (RGPs) are the major neural progenitor cells that generate neurons and glia in the developing mammalian cerebral cortex1-4. In RGPs, the centrosome is positioned away from the nucleus at the apical surface of the ventricular zone of the cerebral cortex5-8. However, the molecular basis and precise function of this distinctive subcellular organization of the centrosome are largely unknown. Here we show in mice that anchoring of the centrosome to the apical membrane controls the mechanical properties of cortical RGPs, and consequently their mitotic behaviour and the size and formation of the cortex. The mother centriole in RGPs develops distal appendages that anchor it to the apical membrane. Selective removal of centrosomal protein 83 (CEP83) eliminates these distal appendages and disrupts the anchorage of the centrosome to the apical membrane, resulting in the disorganization of microtubules and stretching and stiffening of the apical membrane. The elimination of CEP83 also activates the mechanically sensitive yes-associated protein (YAP) and promotes the excessive proliferation of RGPs, together with a subsequent overproduction of intermediate progenitor cells, which leads to the formation of an enlarged cortex with abnormal folding. Simultaneous elimination of YAP suppresses the cortical enlargement and folding that is induced by the removal of CEP83. Together, these results indicate a previously unknown role of the centrosome in regulating the mechanical features of neural progenitor cells and the size and configuration of the mammalian cerebral cortex.

PARD3 dysfunction in conjunction with dynamic HIPPO signaling drives cortical enlargement with massive heterotopia.

Proper organization and orderly mitosis of radial glial progenitors (RGPs) drive the formation of a laminated mammalian cortex in the correct size. However, the molecular underpinnings of the intricate process remain largely unclear. Here we show that RGP behavior and cortical development are controlled by temporally distinct actions of partitioning-defective 3 (PARD3) in concert with dynamic HIPPO signaling. RGPs lacking PARD3 exhibit developmental stage-dependent abnormal switches in division mode, resulting in an initial overproduction of RGPs located largely outside the ventricular zone at the expense of deep-layer neurons. Ectopically localized RGPs subsequently undergo accelerated and excessive neurogenesis, leading to the formation of an enlarged cortex with massive heterotopia and increased seizure susceptibility. Simultaneous removal of HIPPO pathway effectors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) suppresses cortical enlargement and heterotopia formation. These results define a dynamic regulatory program of mammalian cortical development and highlight a progenitor origin of megalencephaly with ribbon heterotopia and epilepsy.

A diffusion MRI-based spatiotemporal continuum of the embryonic mouse brain for probing gene-neuroanatomy connections.

The embryonic mouse brain undergoes drastic changes in establishing basic anatomical compartments and laying out major axonal connections of the developing brain. Correlating anatomical changes with gene-expression patterns is an essential step toward understanding the mechanisms regulating brain development. Traditionally, this is done in a cross-sectional manner, but the dynamic nature of development calls for probing gene-neuroanatomy interactions in a combined spatiotemporal domain. Here, we present a four-dimensional (4D) spatiotemporal continuum of the embryonic mouse brain from E10.5 to E15.5 reconstructed from diffusion magnetic resonance microscopy (dMRM) data. This study achieved unprecedented high-definition dMRM at 30- to 35-µm isotropic resolution, and together with computational neuroanatomy techniques, we revealed both morphological and microscopic changes in the developing brain. We transformed selected gene-expression data to this continuum and correlated them with the dMRM-based neuroanatomical changes in embryonic brains. Within the continuum, we identified distinct developmental modes comprising regional clusters that shared developmental trajectories and similar gene-expression profiles. Our results demonstrate how this 4D continuum can be used to examine spatiotemporal gene-neuroanatomical interactions by connecting upstream genetic events with anatomical changes that emerge later in development. This approach would be useful for large-scale analysis of the cooperative roles of key genes in shaping the developing brain.

In vivo mapping of hippocampal venous vasculature and oxygenation using susceptibility imaging at 7T.

Mapping the small venous vasculature of the hippocampus in vivo is crucial for understanding how functional changes of hippocampus evolve with age. Oxygen utilization in the hippocampus could serve as a sensitive biomarker for early degenerative changes, surpassing hippocampal tissue atrophy as the main source of information regarding tissue degeneration. Using an ultrahigh field (7T) susceptibility-weighted imaging (SWI) sequence, it is possible to capture oxygen-level dependent contrast of submillimeter-sized vessels. Moreover, the quantitative susceptibility mapping (QSM) results derived from SWI data allow for the simultaneous estimation of venous oxygenation levels, thereby enhancing the understanding of hippocampal function. In this study, we proposed two potential imaging markers in a cohort of 19 healthy volunteers aged between 20 and 74 years. These markers were: 1) hippocampal venous density on SWI images and 2) venous susceptibility (Δχvein) in the hippocampus-associated draining veins (the inferior ventricular veins (IVV) and the basal veins of Rosenthal (BVR) using QSM images). They were chosen specifically to help characterize the oxygen utilization of the human hippocampus and medial temporal lobe (MTL). As part of the analysis, we demonstrated the feasibility of measuring hippocampal venous density and Δχvein in the IVV and BVR at 7T with high spatial resolution (0.25 × 0.25 × 1 mm3). Our results demonstrated the in vivo reconstruction of the hippocampal venous system, providing initial evidence regarding the presence of the venous arch structure within the hippocampus. Furthermore, we evaluated the age effect of the two quantitative estimates and observed a significant increase in Δχvein for the IVV with age (p=0.006, r2 = 0.369). This may suggest the potential application of Δχvein in IVV as a marker for assessing changes in atrophy-related hippocampal oxygen utilization in normal aging and neurodegenerative diseases such as AD and dementia.

Improved reconstruction of crossing fibers in the mouse optic pathways with orientation distribution function fingerprinting.

PURPOSE: The accuracy of diffusion MRI tractography reconstruction decreases in the white matter regions with crossing fibers. The optic pathways in rodents provide a challenging structure to test new diffusion tractography approaches because of the small crossing volume within the optic chiasm and the unbalanced 9:1 proportion between the contra- and ipsilateral neural projections from the retina to the lateral geniculate nucleus, respectively. METHODS: Common approaches based on Orientation Distribution Function (ODF) peak finding or statistical inference were compared qualitatively and quantitatively to ODF Fingerprinting (ODF-FP) for reconstruction of crossing fibers within the optic chiasm using in vivo diffusion MRI ( n = 18 $$ n=18 $$ healthy C57BL/6 mice). Manganese-Enhanced MRI (MEMRI) was obtained after intravitreal injection of manganese chloride and used as a reference standard for the optic pathway anatomy. RESULTS: ODF-FP outperformed by over 100% all the tested methods in terms of the ratios between the contra- and ipsilateral segments of the reconstructed optic pathways as well as the spatial overlap between tractography and MEMRI. CONCLUSION: In this challenging model system, ODF-Fingerprinting reduced uncertainty of diffusion tractography for complex structural formations of fiber bundles.

Vascular Aging in the Choroid Plexus: A 7T Ultrasmall Superparamagnetic Iron Oxide (USPIO)-MRI Study.

BACKGROUND: The choroid plexus (ChP), a densely vascularized structure, has drawn increasing attention for its involvement in brain homeostasis and waste clearance. While the volumetric changes have been explored in many imaging studies, few studies have investigated the vascular degeneration associated with aging in the ChP. PURPOSE: To investigate the sub-structural characteristics of the ChP, particularly the vascular compartment using high-resolution 7T imaging enhanced with Ferumoxytol, an ultrasmall super-paramagnetic iron oxide, which greatly increase the susceptibility contrast for vessels. STUDY TYPE: Prospective. SUBJECTS: Forty-nine subjects without neurological disorders (age: 21-80 years; 42 ± 17 years; 20 females). FIELD STRENGTH/SEQUENCE: 7-T with 2D and 3D T2* GRE, 3D MPRAGE T1, 2D TSE T2, and 2D FLAIR. ASSESSMENT: The vascular and stromal compartments of the ChP were segmented using K-means clustering on post-contrast 2D GRE images. Visual and qualitative assessment of ChP vascular characteristics were conducted independently by three observers. Vascular density (Volvessel/VolChP ratio) and susceptibility change (Δχ) induced by Ferumoxytol were analyzed on 3D GRE-derived susceptibility-weighted imaging and quantitative susceptibility mapping, respectively. STATISTICAL TESTS: Independent t-test, Mann-Whitney U test, and Chi-square test were utilized for group comparisons. The relationship between age and ChP's vascular alterations was examined using Pearson's correlation. Intra-class coefficient was calculated for inter-observer agreement. A P value <0.05 was considered statistically significant. RESULTS: 2D GRE images demonstrated superior contrast and accurate delineation of ChP substructures (ICC = 0.86). Older subjects exhibited a significantly smaller vascular density (16.5 ± 4.34%) and lower Δχ (22.10 ± 12.82 ppb) compared to younger subjects (24.85 ± 6.84% and 34.64 ± 12.69 ppb). Vascular density and mean Δχ within the ChP negatively correlated with age (r = -0.48, and r = -0.45). DATA CONCLUSION: Ferumoxytol-enhanced 7T images can demonstrate ChP alterations in elderly with decreased vascular density and expansion of nonvascular compartment. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2.

Using mesoscopic tract-tracing data to guide the estimation of fiber orientation distributions in the mouse brain from diffusion MRI.

Diffusion MRI (dMRI) tractography is the only tool for non-invasive mapping of macroscopic structural connectivity over the entire brain. Although it has been successfully used to reconstruct large white matter tracts in the human and animal brains, the sensitivity and specificity of dMRI tractography remained limited. In particular, the fiber orientation distributions (FODs) estimated from dMRI signals, key to tractography, may deviate from histologically measured fiber orientation in crossing fibers and gray matter regions. In this study, we demonstrated that a deep learning network, trained using mesoscopic tract-tracing data from the Allen Mouse Brain Connectivity Atlas, was able to improve the estimation of FODs from mouse brain dMRI data. Tractography results based on the network generated FODs showed improved specificity while maintaining sensitivity comparable to results based on FOD estimated using a conventional spherical deconvolution method. Our result is a proof-of-concept of how mesoscale tract-tracing data can guide dMRI tractography and enhance our ability to characterize brain connectivity.

Towards reliable reconstruction of the mouse brain corticothalamic connectivity using diffusion MRI.

Diffusion magnetic resonance imaging (dMRI) tractography has yielded intriguing insights into brain circuits and their relationship to behavior in response to gene mutations or neurological diseases across a number of species. Still, existing tractography approaches suffer from limited sensitivity and specificity, leading to uncertain interpretation of the reconstructed connections. Hence, in this study, we aimed to optimize the imaging and computational pipeline to achieve the best possible spatial overlaps between the tractography and tracer-based axonal projection maps within the mouse brain corticothalamic network. We developed a dMRI-based atlas of the mouse forebrain with structural labels imported from the Allen Mouse Brain Atlas (AMBA). Using the atlas and dMRI tractography, we first reconstructed detailed node-to-node mouse brain corticothalamic structural connectivity matrices using different imaging and tractography parameters. We then investigated the effects of each condition for accurate reconstruction of the corticothalamic projections by quantifying the similarities between the tractography and the tracer data from the Allen Mouse Brain Connectivity Atlas (AMBCA). Our results suggest that these parameters significantly affect tractography outcomes and our atlas can be used to investigate macroscopic structural connectivity in the mouse brain. Furthermore, tractography in mouse brain gray matter still face challenges and need improved imaging and tractography methods.

Measuring water exchange on a preclinical MRI system using filter exchange and diffusion time dependent kurtosis imaging.

PURPOSE: Filter exchange imaging (FEXI) and diffusion time (t)-dependent diffusion kurtosis imaging (DKI(t)) are both sensitive to water exchange between tissue compartments. The restrictive effects of tissue microstructure, however, introduce bias to the exchange rate obtained by these two methods, as their interpretation conventionally rely on the Kärger model of barrier limited exchange between Gaussian compartments. Here, we investigated whether FEXI and DKI(t) can provide comparable exchange rates in ex vivo mouse brains. THEORY AND METHODS: FEXI and DKI(t) data were acquired from ex vivo mouse brains on a preclinical MRI system. Phase cycling and negative slice prewinder gradients were used to minimize the interferences from imaging gradients. RESULTS: In the corpus callosum, apparent exchange rate (AXR) from FEXI correlated with the exchange rate (the inverse of exchange time, 1/τex ) from DKI(t) along the radial direction. In comparison, discrepancies between FEXI and DKI(t) were found in the cortex due to low filter efficiency and confounding effects from tissue microstructure. CONCLUSION: The results suggest that FEXI and DKI(t) are sensitive to the same exchange processes in white matter when separated from restrictive effects of microstructure. The complex microstructure in gray matter, with potential exchange among multiple compartments and confounding effects of microstructure, still pose a challenge for FEXI and DKI(t).

Microstructural Alterations in Projection and Association Fibers in Neonatal Hypoxia-Ischemia.

BACKGROUND: Diffusion MRI (dMRI) is known to be sensitive to hypoxic-ischemic encephalopathy (HIE). However, existing dMRI studies used simple diffusion tensor metrics and focused only on a few selected cerebral regions, which cannot provide a comprehensive picture of microstructural injury. PURPOSE: To systematically characterize the microstructural alterations in mild, moderate, and severe HIE neonates compared to healthy neonates with advanced dMRI using region of interest (ROI), tract, and fixel-based analyses. STUDY TYPE: Prospective. POPULATION: A total of 42 neonates (24 males and 18 females). FIELD STRENGTH/SEQUENCE: 3-T, diffusion-weighted echo-planar imaging. ASSESSMENT: Fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD), fiber density (FD), fiber cross-section (FC), and fiber density and cross-section (FDC) were calculated in 40 ROIs and 6 tracts. Fixel-based analysis was performed to assess group differences in individual fiber components within a voxel (fixel). STATISTICAL TESTS: One-way analysis of covariance (ANCOVA) to compare dMRI metrics among severe/moderate/mild HIE and control groups and general linear model for fixel-wise group differences (age, sex, and body weight as covariates). Adjusted P value < 0.05 was considered statistically significant. RESULTS: For severe HIE, ROI-based analysis revealed widespread regions, including the deep nuclei and white matter with reduced FA, while in moderate injury, only FC was decreased around the posterior watershed zones. Tract-based analysis demonstrated significantly reduced FA, FD, and FC in the right inferior fronto-occipital fasciculus (IFOF), right inferior longitudinal fasciculus (ILF), and splenium of corpus callosum (SCC) in moderate HIE, and in right IFOF and left anterior thalamic radiation (ATR) in mild HIE. Correspondingly, we found altered fixels in the right middle-posterior IFOF and ILF, and in the central-to-right part of SCC in moderate HIE. DATA CONCLUSION: For severe HIE, extensive microstructural injury was identified. For moderate-mild HIE, association fiber injury in posterior watershed area with a rightward lateralization was found. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 3.

Early adversity changes the economic conditions of mouse structural brain network organization.

Early adversity can change educational, cognitive, and mental health outcomes. However, the neural processes through which early adversity exerts these effects remain largely unknown. We used generative network modeling of the mouse connectome to test whether unpredictable postnatal stress shifts the constraints that govern the organization of the structural connectome. A model that trades off the wiring cost of long-distance connections with topological homophily (i.e., links between regions with shared neighbors) generated simulations that successfully replicate the rodent connectome. The imposition of early life adversity shifted the best-performing parameter combinations toward zero, heightening the stochastic nature of the generative process. Put simply, unpredictable postnatal stress changes the economic constraints that reproduce rodent connectome organization, introducing greater randomness into the development of the simulations. While this change may constrain the development of cognitive abilities, it could also reflect an adaptive mechanism that facilitates effective responses to future challenges.

Inhomogeneous magnetization transfer MRI of white matter structures in the hypomyelinated shiverer mouse brain.

PURPOSE: Inhomogeneous magnetization transfer (ihMT) MRI is uniquely sensitive to myelin with lipids as a primary source of its contrast. In this study, we investigated whether ihMT can detect white matter structures in the hypomyelinated shiverer mouse brain, a model of dysmyelination. METHODS: Conventional MT and ihMT images were acquired from ex vivo Rag2-/- control and shiverer mouse brains at 7T using previously reported optimized saturation parameters. RESULTS: ihMT ratio (ihMTR) maps revealed hypomyelinated corpus callosum in the shiverer mouse brain, whereas conventional MT ratio (MTR) maps showed no clear contrast. The ihMTR values of the corpus callosum in the shiverer mice were reduced by approximately 40% compared to controls, but remained significantly higher than the ihMTR values of the cortex. CONCLUSION: The finding further confirms ihMT's high myelin specificity and suggests its use as a marker to detect early myelination or myelin repair.

Imaging meningeal inflammation in CNS autoimmunity identifies a therapeutic role for BTK inhibition.

Leptomeningeal inflammation in multiple sclerosis is associated with worse clinical outcomes and greater cortical pathology. Despite progress in identifying this process in multiple sclerosis patients using post-contrast fluid-attenuated inversion recovery imaging, early trials attempting to target meningeal inflammation have been unsuccessful. There is a lack of appropriate model systems to screen potential therapeutic agents targeting meningeal inflammation. We utilized ultra-high field (11.7 T) MRI to perform post-contrast imaging in SJL/J mice with experimental autoimmune encephalomyelitis induced via immunization with proteolipid protein peptide (PLP139-151) and complete Freund's adjuvant. Imaging was performed in both a cross-sectional and longitudinal fashion at time points ranging from 2 to 14 weeks post-immunization. Following imaging, we euthanized animals and collected tissue for pathological evaluation, which revealed dense cellular infiltrates corresponding to areas of contrast enhancement involving the leptomeninges. These areas of meningeal inflammation contained B cells (B220+), T cells (CD3+) and myeloid cells (Mac2+). We also noted features consistent with tertiary lymphoid tissue within these areas, namely the presence of peripheral node addressin-positive structures, C-X-C motif chemokine ligand-13 (CXCL13)-producing cells and FDC-M1+ follicular dendritic cells. In the cortex adjacent to areas of meningeal inflammation we identified astrocytosis, microgliosis, demyelination and evidence of axonal stress/damage. Since areas of meningeal contrast enhancement persisted over several weeks in longitudinal experiments, we utilized this model to test the effects of a therapeutic intervention on established meningeal inflammation. We randomized mice with evidence of meningeal contrast enhancement on MRI scans performed at 6 weeks post-immunization, to treatment with either vehicle or evobrutinib [a Bruton tyrosine kinase (BTK) inhibitor] for a period of 4 weeks. These mice underwent serial imaging; we examined the effect of treatment on the areas of meningeal contrast enhancement and noted a significant reduction in the evobrutinib group compared to vehicle (30% reduction versus 5% increase; P = 0.003). We used ultra-high field MRI to identify areas of meningeal inflammation and to track them over time in SJL/J mice with experimental autoimmune encephalomyelitis, and then used this model to identify BTK inhibition as a novel therapeutic approach to target meningeal inflammation. The results of this study provide support for future studies in multiple sclerosis patients with imaging evidence of meningeal inflammation.

SuperDTI: Ultrafast DTI and fiber tractography with deep learning.

PURPOSE: To develop a deep learning-based reconstruction framework for ultrafast and robust diffusion tensor imaging and fiber tractography. METHODS: SuperDTI was developed to learn the nonlinear relationship between DWIs and the corresponding diffusion tensor parameter maps. It bypasses the tensor fitting procedure, which is highly susceptible to noises and motions in DWIs. The network was trained and tested using data sets from the Human Connectome Project and patients with ischemic stroke. Results from SuperDTI were compared against widely used methods for tensor parameter estimation and fiber tracking. RESULTS: Using training and testing data acquired using the same protocol and scanner, SuperDTI was shown to generate fractional anisotropy and mean diffusivity maps, as well as fiber tractography, from as few as six raw DWIs, with a quantification error of less than 5% in all white-matter and gray-matter regions of interest. It was robust to noises and motions in the testing data. Furthermore, the network trained using healthy volunteer data showed no apparent reduction in lesion detectability when directly applied to stroke patient data. CONCLUSIONS: Our results demonstrate the feasibility of superfast DTI and fiber tractography using deep learning with as few as six DWIs directly, bypassing tensor fitting. Such a significant reduction in scan time may allow the inclusion of DTI into the clinical routine for many potential applications.

Acceleration of three-dimensional diffusion magnetic resonance imaging using a kernel low-rank compressed sensing method.

Diffusion Magnetic Resonance Imaging (dMRI) has shown great potential in probing tissue microstructure and structural connectivity in the brain but is often limited by the lengthy scan time needed to sample the diffusion profile by acquiring multiple diffusion weighted images (DWIs). Although parallel imaging technique has improved the speed of dMRI acquisition, attaining high resolution three dimensional (3D) dMRI on preclinical MRI systems remained still time consuming. In this paper, kernel principal component analysis, a machine learning approach, was employed to estimate the correlation among DWIs. We demonstrated the feasibility of such correlation estimation from low-resolution training DWIs and used the correlation as a constraint to reconstruct high-resolution DWIs from highly under-sampled k-space data, which significantly reduced the scan time. Using full k-space 3D dMRI data of post-mortem mouse brains, we retrospectively compared the performance of the so-called kernel low rank (KLR) method with a conventional compressed sensing (CS) method in terms of image quality and ability to resolve complex fiber orientations and connectivity. The results demonstrated that the KLR-CS method outperformed the conventional CS method for acceleration factors up to 8 and was likely to enhance our ability to investigate brain microstructure and connectivity using high-resolution 3D dMRI.

Mean Diffusivity in Striatum Correlates With Acute Neuronal Death but Not Lesser Neuronal Injury in a Pilot Study of Neonatal Piglets With Encephalopathy.

BACKGROUND: Diffusion MRI is routinely used to evaluate brain injury in neonatal encephalopathy. Although abnormal mean diffusivity (MD) is often attributed to cytotoxic edema, the specific contribution from neuronal pathology is unclear. PURPOSE: To determine whether MD from high-resolution diffusion tensor imaging (DTI) can detect variable degrees of neuronal degeneration and pathology in piglets with brain injury induced by excitotoxicity or global hypoxia-ischemia (HI) with or without overt infarction. STUDY TYPE: Prospective. ANIMAL MODEL: Excitotoxic brain injury was induced in six neonatal piglets by intrastriatal stereotaxic injection of the glutamate receptor agonist quinolinic acid (QA). Three piglets underwent global HI or a sham procedure. Piglets recovered for 20-96 hours before undergoing MRI (n = 9). FIELD STRENGTH/SEQUENCE: 3.0T MRI with DTI, T1 - and T2 -weighted imaging. ASSESSMENT: MD, fractional anisotropy (FA), and qualitative T2 injury were assessed in the putamen and caudate. The cell bodies of normal neurons, degenerating neurons (excitotoxic necrosis, ischemic necrosis, or necrosis-apoptosis cell death continuum), and injured neurons with equivocal degeneration were counted by histopathology. STATISTICAL TESTS: Spearman correlations were used to compare MD and FA to normal, degenerating, and injured neurons. T2 injury and neuron counts were evaluated by descriptive analysis. RESULTS: The QA insult generated titratable levels of neuronal pathology. In QA, HI, and sham piglets, lower MD correlated with higher ratios of degenerating-to-total neurons (P < 0.05), lower ratios of normal-to-total neurons (P < 0.05), and greater numbers of degenerating neurons (P < 0.05). MD did not correlate with abnormal neurons exhibiting nascent injury (P > 0.99). Neuron counts were not related to FA (P > 0.30) or to qualitative injury from T2 -weighted MRI. DATA CONCLUSION: MD is more accurate than FA for detecting neuronal degeneration and loss during acute recovery from neonatal excitotoxic and HI brain injury. MD does not reliably detect nonfulminant, nascent, and potentially reversible neuronal injury. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2 J. Magn. Reson. Imaging 2020;52:1216-1226.

Canonical Wnt signaling regulates patterning, differentiation and nucleogenesis in mouse hypothalamus and prethalamus.

The hypothalamus is a small, but anatomically and functionally complex region of the brain whose development is poorly understood. In this study, we have explored its development by studying the canonical Wnt signaling pathway, generating gain and loss of function mutations of beta-catenin (Ctnnb1) in both hypothalamic and prethalamic neuroepithelium. Deletion of Ctnnb1 resulted in an anteriorized and hypoplastic hypothalamus. Posterior structures were lost or reduced, and anterior structures were expanded. In contrast, overexpression of a constitutively active mutant form of Ctnnb1 resulted in severe hyperplasia of prethalamus and hypothalamus, and expanded expression of a subset of posterior and premamillary hypothalamic markers. Moderate defects in differentiation of Arx-positive GABAergic neural precursors were observed in both prethalamus and hypothalamus of Ctnnb1 loss of function mutants, while in gain of function mutants, their differentiation was completely suppressed, although markers of prethalamic progenitors were preserved. Multiple other region-specific markers, including several specific posterior hypothalamic structures, were also suppressed in Ctnnb1 gain of function mutations. Severe, region-specific defects in hypothalamic nucleogenesis were also observed in both gain and loss of function mutations of Ctnnb1. Finally, both gain and loss of function of Ctnnb1 also produced severe, non-cell autonomous disruptions of pituitary development. These findings demonstrate a central and multifaceted role for canonical Wnt signaling in regulating growth, patterning, differentiation and nucleogenesis in multiple diencephalic regions.

Evidence of the diffusion time dependence of intravoxel incoherent motion in the brain.

PURPOSE: To investigate the diffusion time (TD ) dependence of intravoxel incoherent motion (IVIM) signals in the brain. METHODS: A 3-compartment IVIM model was proposed to characterize 2 types of microcirculatory flows in addition to tissue water in the brain: flows that cross multiple vascular segments (pseudo-diffusive) and flows that stay in 1 segment (ballistic) within TD . The model was first evaluated using simulated flow signals. Experimentally, flow-compensated (FC) pulsed-gradient spin-echo (PGSE) and oscillating-gradient spin-echo (OGSE) sequences were tested using a flow phantom and then used to examine IVIM signals in the mouse brain with TD ranging from ~2.5 ms to 40 ms on an 11.7T scanner. RESULTS: By fitting the model to simulated flow signals, we demonstrated the TD dependency of the estimated fraction of pseudo-diffusive flow and the pseudo-diffusion coefficient (D*), which were dictated by the characteristic timescale of microcirculatory flow (τ). Flow phantom experiments validated that the OGSE and FC-PGSE sequences were not susceptible to the change in flow velocity. In vivo mouse brain data showed that both the estimated fraction of pseudo-diffusive flow and D* increased significantly as TD increased. CONCLUSION: We demonstrated that IVIM signals measured in the brain are TD -dependent, potentially because more microcirculatory flows approach the pseudo-diffusive limit as TD increases with respect to τ. Measuring the TD dependency of IVIM signals may provide additional information on microvascular flows in the brain.

Oscillating gradient diffusion kurtosis imaging of normal and injured mouse brains.

Recent advances in diffusion MRI employ multiple diffusion encoding schemes with varying diffusion direction, weighting, and diffusion time to investigate specific microstructural properties in biological tissues. In this study, we examined time-dependent diffusion kurtosis contrast in adult mouse brains and in neonatal mouse brains after hypoxic-ischemic (HI) injury. In vivo diffusion kurtosis maps were acquired with a short diffusion time using an oscillating gradient spin echo (OGSE) sequence at 100 Hz and with a relatively long diffusion time (20 ms) using a pulsed gradient spin echo (PGSE) sequence. In the adult mouse brain, we found that the cortex and hippocampus showed larger differences between OGSE kurtosis and PGSE kurtosis than major white matter tracts. In neonatal mouse brains with unilateral HI injury, the OGSE kurtosis map overall provided stronger edema contrast than the PGSE kurtosis map, and the differences between OGSE and PGSE kurtosis measurements in the edema region reflected heterogeneity of injury. This is the first in vivo study that has demonstrated multi-direction OGSE kurtosis contrasts in the mouse brain. Comparing PGSE and OGSE kurtosis measures may provide additional information on microstructural changes after ischemic stroke.

In vivo assessment of the placental anatomy and perfusion in a mouse model of intrauterine inflammation.

BACKGROUND: Magnetic resonance imaging (MRI) provides useful markers to examine placental function. MRI features of placental injury due to intrauterine inflammation-a common risk during pregnancy, are not well known. PURPOSE: To investigate the capability of structural MRI and intravoxel incoherent motion (IVIM) imaging in examining acute placental injury in a mouse model of intrauterine inflammation, as well as gestation-dependent placental changes. STUDY TYPE: Prospective study. ANIMAL MODEL: Pregnant CD1 mice were scanned on embryonic day 15 (E15, n = 40 placentas from six dams) and E17. On E17, mice were subjected to intrauterine injury by exposure to lipopolysaccharide (LPS, n = 25 placentas from three dams) or sham injury (n = 25 placentas from three dams). FIELD STRENGTH/SEQUENCE: In vivo MRI was performed on an 11.7T Bruker scanner, using a fast spin-echo sequence and a diffusion-weighted echo-planar imaging (EPI) sequence. ASSESSMENT: T2 -weighted MRI was acquired to evaluate placental volume. IVIM imaging was performed in a restricted field-of-view using 15 b-values from 10-800 s/mm2 , based on which, the pseudodiffusion fraction (f), pseudodiffusion coefficient (D*), and tissue water coefficient (D) were estimated with a two-step fitting procedure. STATISTICAL TESTS: Two-way analysis of variance (ANOVA) was used to evaluate the group differences. RESULTS: The placental volume increased by ∼21% from E15 to E17 (P < 0.01), and a 15% volume loss was observed at 6 hours after LPS exposure (P < 0.01). IVIM parameters (f, D*, and f·D*) were similar between the E15 and E17 sham groups (P > 0.05), which was significantly reduced in the LPS-exposed placentas compared to the shams (P < 0.001). D values decreased from E15 to E17 (P < 0.05), which were further reduced after LPS exposure (P < 0.05). Changes in placental area and vascular density were histologically identified in the LPS-exposed group, along with gestation-dependent changes. DATA CONCLUSION: Our results suggested structural MRI and IVIM measurements are potential markers for detecting acute placental injury after intrauterine inflammation. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:1260-1267.