In utero localized diffusion MRI of the embryonic mouse brain microstructure and injury

Longitudinal in Utero Analysis of Engrailed-1 Knockout Mouse Embryonic Phenotypes Using High-Frequency Ultrasound

Large-scale international efforts to generate and analyze loss-of-function mutations in each of the approximately 20,000 protein-encoding gene mutations are ongoing using the "knockout" mouse as a model organism. Because one-third of gene knockouts are expected to result in embryonic lethality, it is important to develop non-invasive in utero imaging methods to detect and monitor mutant phenotypes in mouse embryos. We describe the utility of 3-D high-frequency (40-MHz) ultrasound (HFU) for longitudinal in utero imaging of mouse embryos between embryonic days (E) 11.5 and E14.5, which represent critical stages of brain and organ development. Engrailed-1 knockout (En1-ko) mouse embryos and their normal control littermates were imaged with HFU in 3-D, enabling visualization of morphological phenotypes in the developing brains, limbs and heads of the En1-ko embryos. Recently developed deep learning approaches were used to automatically segment the embryonic brain ventricles and bodies from the 3-D HFU images, allowing quantitative volumetric analyses of the En1-ko brain phenotypes. Taken together, these results show great promise for the application of longitudinal 3-D HFU to analyze knockout mouse embryos in utero.

Diffusion-weighted magnetic resonance imaging in rat kidney using two-dimensional navigated, interleaved echo-planar imaging at 7.0 T

The purpose of this study was to investigate the feasibility of two-dimensional (2D) navigated, interleaved multishot echo-planar imaging (EPI) to enhance kidney diffusion-weighted imaging (DWI) in rats at 7.0 T. Fully sampled interleaved four-shot EPI with 2D navigators was tailored for kidney DWI (Sprague-Dawley rats, n = 7) on a 7.0-T small bore preclinical scanner. The image quality of four-shot EPI was compared with T₂ -weighted rapid acquisition with relaxation enhancement (RARE) (reference) and single-shot EPI (ss-EPI) without and with parallel imaging (PI). The contrast-to-noise ratio (CNR) was examined to assess the image quality for the EPI approaches. The Dice similarity coefficient and the Hausdorff distance were used for evaluation of image distortion. Mean diffusivity (MD) and fractional anisotropy (FA) were calculated for renal cortex and medulla for all DWI approaches. The corticomedullary difference of MD and FA were assessed by Wilcoxon signed-rank test. Four-shot EPI showed the highest CNR among the three EPI variants and lowest geometric distortion versus T₂ -weighted RARE (mean Dice: 0.77 for ss-EPI without PI, 0.88 for ss-EPI with twofold undersampling, and 0.92 for four-shot EPI). The FA map derived from four-shot EPI clearly identified a highly anisotropic region corresponding to the inner stripe of the outer medulla. Four-shot EPI successfully discerned differences in both MD and FA between renal cortex and medulla. In conclusion, 2D navigated, interleaved multishot EPI facilitates high-quality rat kidney DWI with clearly depicted intralayer and interlayer structure and substantially reduced image distortion. This approach enables the anatomic integrity of DWI-MRI in small rodents and has the potential to benefit the characterization of renal microstructure in preclinical studies.

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

We established an ultra high-resolution diffusion MRI atlas of the embryonic mouse brains from E10.5 to E15.5, which characterizes the continuous changes of brain morphology and microstructures at mesoscopic scale. By integrating gene-expression data into the spatiotemporal continuum, we can navigate the evolving landscape of gene expression and neuroanatomy across both spatial and temporal dimensions to visualize their interactions in normal and abnormal embryonic brain development. We also identified regional clusters with distinct developmental trajectories and identified gene-expression profiles that matched to these regional domains. The diffusion MRI–based continuum of the embryonic brain and the computational techniques presented in this study offer a valuable tool for systematic study of the genetic control of brain development.

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.

The physiological basis underlying functional connectivity differences in older adults: A multi-modal analysis of resting-state fMRI

The purpose of this study was to determine if differences in functional connectivity strength (FCS) with age were confounded by vascular parameters including resting cerebral blood flow (CBF₀), cerebrovascular reactivity (CVR), and BOLD-CBF coupling. Neuroimaging data were collected from 13 younger adults (24 ± 2 years) and 14 older adults (71 ± 4 years). A dual-echo resting state pseudo-continuous arterial spin labeling sequence was performed, as well as a BOLD breath-hold protocol. A group independent component analysis was used to identify networks, which were amalgamated into a region of interest (ROI). Within the ROI, FC strength (FCS) was computed for all voxels and compared across the groups. CBF₀, CVR and BOLD-CBF coupling were examined within voxels where FCS was different between young and older adults. FCS was greater in old compared to young (P = 0.001). When the effect of CBF₀, CVR and BOLD-CBF coupling on FCS was examined, BOLD-CBF coupling had a significant effect (P = 0.003) and group differences in FCS were not present once all vascular parameters were considered in the statistical model (P = 0.07). These findings indicate that future studies of FCS should consider vascular physiological markers in order to improve our understanding of aging processes on brain connectivity.

sEVsRVG selectively delivers antiviral siRNA to fetus brain, inhibits ZIKV infection and mitigates ZIKV-induced microcephaly in mouse model

Zika virus (ZIKV), a flavivirus associated with neurological disorders, constitutes a global health threat. During pregnancy, ZIKV traverses the placenta and causes congenital disease such as microcephaly and Guillain-Barré syndrome in newborns. To develop a specific antiviral therapy against ZIKV-induced microcephaly that could cross placental and blood-brain barriers, we designed targeted small extracellular vesicles (sEVs) encapsulating antiviral siRNA (small interfering RNA) to inhibit ZIKV. The neuro-specific targeting was achieved by engineering EVs membrane protein lamp2b fused with a neuron-specific rabies virus glycoprotein derived peptide (RVG). Intravenous administration of the RVG-engineered sEVs loaded with siRNA (ZIKV-specific siRNA) protected pregnant AG6 mice against vertical transmission of ZIKV. Particularly, sEVs^RVG-siRNA traversed placental and blood-brain barriers and suppressed ZIKV infection in fetal brains. Moreover, sEVs^RVG-siRNA alleviated the neuroinflammation and neurological damage caused by ZIKV in the fetal mouse model. In general, we developed a sEVs-based targeted system of antiviral therapy for brain and fetal brain infections.

Type 1 Cytotoxic T Cells Increase in Placenta after Intrauterine Inflammation

CD8+ T cells recognize non-self antigen by MHC class I molecules and kill the target cells by the release of proinflammatory cytokines such as interferon gamma (IFN-γ) and tumor necrosis factor alpha (TNF-α). Our group previously reported an increase of CD8+ T‐cell trafficking in the placenta with exposure to Lipopolysaccharides (LPS). CD8+ cytotoxic T cells have been classified into distinct subsets based upon cytokine production: Tc1 cells produce IFN-γ, Tc2 cells produce interleukin 4 (IL-4). Accordingly, the purpose of this research is to analyze the subsets of placenta CD8+ T cells. We hypothesized that LPS injection would induce a change of properties of CD8+ T cell and Tc1/Tc2 ratio. We investigated the subsets of CD8+ T cell infiltration to placenta and their specific function in response to LPS-induced inflammation in a mouse model. At embryonic (E) day 17, pregnant CD-1 dams received an intrauterine injection of 25 µg LPS in100 μl PBS or 100 μl of PBS only. Flow cytometry was used to quantify CD8+ T cells, evaluate the phenotype and subtypes, and detect markers of Tc1 and Tc2 cells in placenta, at 6 hours and 24 hours post injection (hpi). Intracellular staining and flow cytometry were performed to characterize cytokines produced by CD8+ T cells. Standard statistical analysis were employed. After 6 and 24 hours of LPS injection, total CD8 T cells increased (P<0.05). Tc1 cells expanded (P<0.05) in LPS-treated dams compared with the PBS group. The Tc1/Tc2 ratio was significantly higher in the LPS group than the PBS group (P<0.05). The expression of TNF-α and IFN-γ were increased in LPS group both at 6hpi and 24 hpi (P<0.05). We identified functional placental CD8+ T cell subtypes and found a significant increase ratio of Tc1/Tc2. Following IUI, CD8+ T cells induced inflammatory response in the placenta primarily via the production of Type 1 cytokines such as IFN-γ and TNF-α. We have provided evidence of a Tc1-bias response and cytokines in the mouse model of IUI.

Placental Macrophages Demonstrate Sex-Specific Response to Intrauterine Inflammation and May Serve as a Marker of Perinatal Neuroinflammation

Preterm birth (PTB) is considered to be one of the most frequent causes of neonatal death. Prompt and effective measures to predict adverse fetal outcome following PTB are urgently needed. Placenta macrophages are a critical immune cell population during pregnancy, phenotypically divided into M1 and M2 subsets. An established mouse model of intrauterine inflammation (IUI) was applied. Placenta (labyrinth) and corresponding fetal brain were harvested within 24 hours post injection (hpi). Flow cytometry, Western blot, real-time qPCR, and regular histology were utilized to examine the cytokines, macrophage polarization, and sex-specificity. Placental exposure to LPS led to significantly reduced labyrinth thickness compared to PBS-exposed controls as early as 3 hpi, accompanied by apoptosis and necrosis. Pro-inflammatory M1 markers, Il-1β, and iNOS, and anti-inflammatory M2 marker Il-10 increased significantly in placentas exposed to IUI. Analysis of flow cytometry revealed that fetal macrophages (Hofbauer cell, HBCs) were mostly M1-like and that maternal inter-labyrinth macrophages (MIM) were M2-like in their features in IUI. Male fetuses displayed significantly decreased M2-like features in HBCs at 3 and 6 hpi, while female fetuses showed significant increase in M2-like features in MIM at 3 and 6 hpi. Furthermore, there was a significant correlation between the frequency of HBCs and corresponding microglial marker expression at 3 and 6 hpi. Placental macrophages demonstrated sex-specific features in response to IUI. Specifically, HBCs may be a potential biomarker for fetal brain injury at preterm birth.

Deuterium Magnetic Resonance Imaging and the Discrimination of Fetoplacental Metabolism in Normal and L-NAME-Induced Preeclamptic Mice

Recent magnetic resonance studies in healthy and cancerous organs have concluded that deuterated metabolites possess highly desirable properties for mapping non-invasively and, as they happen, characterizing glycolysis and other biochemical processes in animals and humans. A promising avenue of this deuterium metabolic imaging (DMI) approach involves looking at the fate of externally administered ²H_6,6′-glucose, as it is taken up and metabolized into different products as a function of time. This study employs deuterium magnetic resonance to follow the metabolism of wildtype and preeclamptic pregnant mice models, focusing on maternal and fetoplacental organs over ≈2 h post-injection. ²H_6,6′-glucose uptake was observed in the placenta and in specific downstream organs such as the fetal heart and liver. Main metabolic products included ²H_3,3′-lactate and ²H-water, which were produced in individual fetoplacental organs with distinct time traces. Glucose uptake in the organs of most preeclamptic animals appeared more elevated than in the control mice (p = 0.02); also higher was the production of ²H-water arising from this glucose. However, the most notable differences arose in the ²H_3,3′-lactate concentration, which was ca. two-fold more abundant in the placenta (p = 0.005) and in the fetal (p = 0.01) organs of preeclamptic-like animals, than in control mice. This is consistent with literature reports about hypoxic conditions arising in preeclamptic and growth-restricted pregnancies, which could lead to an enhancement in anaerobic glycolysis. Overall, the present measurements suggest that DMI, a minimally invasive approach, may offer new ways of studying and characterizing health and disease in mammalian pregnancies, including humans.

Mapping fetal brain development based on automated segmentation and 4D brain atlasing

Fetal brain MRI has become an important tool for in utero assessment of brain development and disorders. However, quantitative analysis of fetal brain MRI remains difficult, partially due to the limited tools for automated preprocessing and the lack of normative brain templates. In this paper, we proposed an automated pipeline for fetal brain extraction, super-resolution reconstruction, and fetal brain atlasing to quantitatively map in utero fetal brain development during mid-to-late gestation in a Chinese population. First, we designed a U-net convolutional neural network for automated fetal brain extraction, which achieved an average accuracy of 97%. We then generated a developing fetal brain atlas, using an iterative linear and nonlinear registration approach. Based on the 4D spatiotemporal atlas, we quantified the morphological development of the fetal brain between 23 and 36 weeks of gestation. The proposed pipeline enabled the fully automated volumetric reconstruction for clinically available fetal brain MRI data, and the 4D fetal brain atlas provided normative templates for the quantitative characterization of fetal brain development, especially in the Chinese population.

Dose-dependent structural and immunological changes in the placenta and fetal brain in response to systemic inflammation during pregnancy

PROBLEM

Systemic maternal inflammation is associated with adverse neonatal sequelae. We tested the hypothesis that IL-1β is a key inflammatory regulator of adverse pregnancy outcomes.

METHOD OF STUDY

Pregnant mice were treated with intraperitoneal injections of IL-1β (0, 0.1, 0.5, or 1 μg) from embryonic day (E)14 to E17. Placenta and fetal brains were harvested and analyzed for morphologic changes and IL-1β signaling markers.

RESULTS

As compared with non-treated dams, maternal injections with IL-1β resulted in increased p-NF-κB and caspase-1 in placentas and fetal brains, but not consistently in spleens, suggesting induction of intrinsic IL-1β production. These findings were confirmed by increased levels of IL-1β in the placentas of the IL-1β-treated dams. Systemic treatment of dams with IL-1β suppressed Stat1 signaling. Maternal inflammation caused by IL-1β treatment reduced fetal viability to 80.6% and 58.9%, in dams treated with either 0.5 or 1 μg of IL-1β, respectively. In the placentas, there was an IL-1β dose-dependent distortion of the labyrinth structure, decreased numbers of mononuclear trophoblast giant cells, and reduced proportions of endothelial cells as compared to placentas from control dams. In fetal brains collected at E17, there was an IL-1β dose-dependent reduction in cortical neuronal morphology.

CONCLUSION

This work demonstrates that systemic IL-1β injection causes dose-dependent structural and functional changes in the placenta and fetal brain.

High-resolution diffusion MRI studies of development in pregnant mice visualized by novel spatiotemporal encoding schemes

This study introduces an MRI approach to map diffusion of water in vivo with high resolution under challenging conditions; the approach's potential is then used in diffusivity characterizations of embryos and fetoplacental units in pregnant mice, as well as of newborn mice in their initial postnatal period. The method relies on performing self-referenced spatiotemporal encoded MRI acquisitions, which can achieve the motional and susceptibility immunities needed to target challenging regions such as a mouse's abdominal cavity in a single shot. When suitably combined with zooming-in and novel interleaving procedures, these scans can overcome the inhomogeneity and sensitivity challenges arising upon targeting ≈100 μm in-plane resolutions, and thereby enable longitudinal development studies of abdominal organs that have hitherto eluded in vivo diffusion-weighted imaging. This is employed here to follow processes related to embryonic implantation and placentation, including the final stages of mouse gastrulation, the development of white matter in fetal brains, the maturation of fetal spines, and the evolution of the different layers making up mouse hemochorial placentas. The protocol's ability to extract diffusivity information in challenging regions as a function of embryonic mouse development is thus demonstrated, and its usefulness as a tool for visualizing pregnancy-related developmental changes in rodents is discussed.

Placental malperfusion in response to intrauterine inflammation and its connection to fetal sequelae

Exposure to intrauterine inflammation (IUI) is associated with short- and long-term adverse perinatal outcomes. However, little data exist on utilizing placenta to prognosticate fetal injury in this scenario. Our study aimed to utilize imaging modalities to evaluate mechanisms contributing to placental injury following IUI exposure and correlated it to concomitant fetal brain injury. CD1 pregnant dams underwent laparotomies and received intrauterine injections of either lipopolysaccharide (LPS; a model of IUI) or phosphate-buffered saline (PBS). In utero ultrasound Doppler velocimetry of uterine and umbilical arteries and magnetic resonance imaging (MRI) of placental volumes with confirmatory immunohistochemical (vimentin) and histochemistry (fibrin) analyses were performed. ELISA for thrombosis markers, fibrinogen and fibrin was performed to analyze thrombi in placenta. Fetal brain immunohistochemistry was performed to detect microglial activation (ionized calcium-binding adaptor molecule 1, Iba1). On ultrasound, LPS group demonstrated elevated resistance indices, pulsatility indices and a greater occurrence of absent end-diastolic flow in the umbilical and uterine arteries. In the fetus, there was an increased cardiac Tei indices in the LPS group. MRI revealed decreased volume of placenta in the LPS group associated with placental thinning and placental endothelial damage on immunohistochemistry. Decreased fibrinogen content and more thrombi staining in placenta exposed to maternal LPS indicated the hypercoagulability. Furthermore, the expression of Iba1was significantly associated with placental thickness (r = -0.7890, Pearson correlation coefficient). Our data indicate that IUI can trigger events leading to maternal placental malperfusion and fetal vessel resistance, as well as predispose the developing fetus to cardiac dysfunction and brain damage. Furthermore, our data suggest that prenatal ultrasound can be a real-time clinical tool for assessing fetal risk for adverse neurologic outcomes following the potential IUI exposure.

Maternal Glucose Supplementation in a Murine Model of Chorioamnionitis Alleviates Dysregulation of Autophagy in Fetal Brain

Fetal brain injury induced by intrauterine inflammation is a major risk factor for adverse neurological outcomes, including cerebral palsy, cognitive dysfunction, and behavioral disabilities. There are no adequate therapies for neuronal protection to reduce fetal brain injury, especially new strategies that may apply promptly and conveniently. In this study, we explored the effect of maternal glucose administration in a mouse model of intrauterine inflammation at term. Our results demonstrated that maternal glucose supplementation significantly increased survival birth rate and improved the neurobehavioral performance of pups exposed to intrauterine inflammation. Furthermore, we demonstrated that maternal glucose administration improved myelination and oligodendrocyte development in offspring exposed to intrauterine inflammation. Though the maternal blood glucose concentration was temporally prevented from decrease induced by intrauterine inflammation, the glucose concentration in fetal brain was not recovered by maternal glucose supplementation. The adenosine triphosphate (ATP) level and autophagy in fetal brain were regulated by maternal glucose supplementation, which may prevent dysregulation of cellular metabolism. Our study is the first to provide evidence for the role of maternal glucose supplementation in the cell survival of fetal brain during intrauterine inflammation and further support the possible medication with maternal glucose treatment.

In Utero MRI of Mouse Embryos

Genetically engineered mouse models are used extensively as models of human development and developmental diseases. Conventional histological approaches are static and two-dimensional, and do not provide a full understanding of the dynamic, spatiotemporal changes in developing mouse embryos. Magnetic resonance imaging (MRI) offers a noninvasive and longitudinal approach for three-dimensional in utero imaging of normal and mutant mouse embryos. In this chapter, we describe MRI approaches that have been developed for imaging the living embryonic mouse brain and vasculature. Details are provided on the animal preparation and setup, MRI equipment, acquisition and reconstruction methods that have been found to be most useful for in utero MRI, including examples of applications to fetal mouse neuroimaging.

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.

Intrauterine Zika virus infection of pregnant immunocompetent mice models transplacental transmission and adverse perinatal outcomes

Zika virus (ZIKV) crosses the placenta and causes congenital disease. Here we develop an animal model utilizing direct ZIKV inoculation into the uterine wall of pregnant, immunocompetent mice to evaluate transplacental transmission. Intrauterine inoculation at embryonic day (E) 10, but not E14, with African, Asian or American strains of ZIKV reduces fetal viability and increases infection of placental and fetal tissues. ZIKV inoculation at E10 causes placental inflammation, placental dysfunction and reduces neonatal brain cortical thickness, which is associated with increased activation of microglia. Viral antigen localizes in trophoblast and endothelial cells in the placenta, and endothelial, microglial and neural progenitor cells in the fetal brain. ZIKV infection of the placenta increases production of IFNβ and expression of IFN-stimulated genes 48 h after infection. This mouse model provides a platform for identifying factors at the maternal-fetal interface that contribute to adverse perinatal outcomes in a host with an intact immune system.

Recent Progress in Magnetic Resonance Imaging of the Embryonic and Neonatal Mouse Brain

The laboratory mouse has been widely used as a model system to investigate the genetic control mechanisms of mammalian brain development. Magnetic resonance imaging (MRI) is an important tool to characterize changes in brain anatomy in mutant mouse strains and injury progression in mouse models of fetal and neonatal brain injury. Progress in the last decade has enabled us to acquire MRI data with increasing anatomical details from the embryonic and neonatal mouse brain. High-resolution ex vivo MRI, especially with advanced diffusion MRI methods, can visualize complex microstructural organizations in the developing mouse brain. In vivo MRI of the embryonic mouse brain, which is critical for tracking anatomical changes longitudinally, has become available. Applications of these techniques may lead to further insights into the complex and dynamic processes of brain development.