In vivo high-resolution diffusion tensor imaging of the developing neonatal rat cortex and its relationship to glial and dendritic maturation

A Cellular Ground Truth to Develop MRI Signatures in Glioma Models by Correlative Light Sheet Microscopy and Atlas-Based Coregistration

Glioblastoma is the most common malignant primary brain tumor with poor overall survival. Magnetic resonance imaging (MRI) is the main imaging modality for glioblastoma but has inherent shortcomings. The molecular and cellular basis of MR signals is incompletely understood. We established a ground truth-based image analysis platform to coregister MRI and light sheet microscopy (LSM) data to each other and to an anatomic reference atlas for quantification of 20 predefined anatomic subregions. Our pipeline also includes a segmentation and quantification approach for single myeloid cells in entire LSM datasets. This method was applied to three preclinical glioma models in male and female mice (GL261, U87MG, and S24), which exhibit different key features of the human glioma. Multiparametric MR data including T2-weighted sequences, diffusion tensor imaging, T2 and T2* relaxometry were acquired. Following tissue clearing, LSM focused on the analysis of tumor cell density, microvasculature, and innate immune cell infiltration. Correlated analysis revealed differences in quantitative MRI metrics between the tumor-bearing and the contralateral hemisphere. LSM identified tumor subregions that differed in their MRI characteristics, indicating tumor heterogeneity. Interestingly, MRI signatures, defined as unique combinations of different MRI parameters, differed greatly between the models. The direct correlation of MRI and LSM allows an in-depth characterization of preclinical glioma and can be used to decipher the structural, cellular, and, likely, molecular basis of tumoral MRI biomarkers. Our approach may be applied in other preclinical brain tumor or neurologic disease models, and the derived MRI signatures could ultimately inform image interpretation in a clinical setting.SIGNIFICANCE STATEMENT We established a histologic ground truth-based approach for MR image analyses and tested this method in three preclinical glioma models exhibiting different features of glioblastoma. Coregistration of light sheet microscopy to MRI allowed for an evaluation of quantitative MRI data in histologically distinct tumor subregions. Coregistration to a mouse brain atlas enabled a regional comparison of MRI parameters with a histologically informed interpretation of the results. Our approach is transferable to other preclinical models of brain tumors and further neurologic disorders. The method can be used to decipher the structural, cellular, and molecular basis of MRI signal characteristics. Ultimately, information derived from such analyses could strengthen the neuroradiological evaluation of glioblastoma as they enhance the interpretation of MRI data.

Age- and sex-related dynamics of structural and functional motor behavior interactions in striatum neurons in rats

Aim. To study the age-related dynamics of structural and functional interactions of striatal neurons in the implementation of acts of motor behaviour in rats of both sexes.

Materials and methods. The study was carried out on 36 Wistar rats of both sexes aged 2, 7 and 16 months (n = 6 per group). In animals of all groups, locomotor activity was determined using a Laboras device (Metris, the Netherlands) for

15 minutes, after which the brain was sampled to determine the number and size of neurons in the striatum. The median and interquartile range of the index of motor activity and the number of neurons were determined, and to study the relationship between these indicators, a correlation and regression analysis was performed with the construction of linear and polynomial trends, and the coefficient of determination R2 was calculated.

Results. The size of neurons did not change significantly with age in the rats of both sexes. The number of neurons differed statistically in the rats of different sexes in all age groups. In male rats, the maximum number of neurons was noted at the age of 7 months with a decrease to 16 months. In female rats, the maximum number of neurons was recorded at the age of 2 months with a further decrease to 7 and 16 months. According to the regression analysis, a linear strong relationship (R2 =

0.80 for males, R2 = 0.79 for females) was established between the number of neurons in the striatum and motor activity in 2-month-old animals. At the age of 7 and 16 months the relationship is non-linear.

Conclusion. The number of neurons in the striatum is subject to sex and age dynamics, while their size remains unchanged from 2 to 16 months. For animals of both sexes, a decrease in the role of the striatum in providing motor activity in the process of growing up was noted. This relationship reaches its maximum in 2-month-old rats and then decreases.

Association of War Zone-Related Stress With Alterations in Limbic Gray Matter Microstructure

IMPORTANCE

Military service members returning from theaters of war are at increased risk for mental illness, but despite high prevalence and substantial individual and societal burden, the underlying pathomechanisms remain largely unknown. Exposure to high levels of emotional stress in theaters of war and mild traumatic brain injury (mTBI) are presumed factors associated with risk for the development of mental disorders.

OBJECTIVE

To investigate (1) whether war zone-related stress is associated with microstructural alterations in limbic gray matter (GM) independent of mental disorders common in this population, (2) whether associations between war zone-related stress and limbic GM microstructure are modulated by a history of mTBI, and (3) whether alterations in limbic GM microstructure are associated with neuropsychological functioning.

DESIGN, SETTING, AND PARTICIPANTS

This cohort study was part of the TRACTS (Translational Research Center for TBI and Stress Disorders) study, which took place in 2010 to 2014 at the Veterans Affair Rehabilitation Research and Development TBI National Network Research Center. Participants included male veterans (aged 18-65 years) with available diffusion tensor imaging data enrolled in the TRACTS study. Data analysis was performed between December 2017 to September 2021.

EXPOSURES

The Deployment Risk and Resilience Inventory (DRRI) was used to measure exposure to war zone-related stress. The Boston Assessment of TBI-Lifetime was used to assess history of mTBI. Stroop Inhibition (Stroop-IN) and Inhibition/Switching (Stroop-IS) Total Error Scaled Scores were used to assess executive or attentional control functions.

MAIN OUTCOMES AND MEASURES

Diffusion characteristics (fractional anisotropy of tissue [FAT]) of 16 limbic and paralimbic GM regions and measures of functional outcome.

RESULTS

Among 384 male veterans recruited, 168 (mean [SD] age, 31.4 [7.4] years) were analyzed. Greater war zone-related stress was associated with lower FAT in the cingulate (DRRI-combat left: P = .002, partial r = -0.289; DRRI-combat right: P = .02, partial r = -0.216; DRRI-aftermath left: P = .004, partial r = -0.281; DRRI-aftermath right: P = .02, partial r = -0.219), orbitofrontal (DRRI-combat left medial orbitofrontal cortex: P = .02, partial r = -0.222; DRRI-combat right medial orbitofrontal cortex: P = .005, partial r = -0.256; DRRI-aftermath left medial orbitofrontal cortex: P = .02, partial r = -0.214; DRRI-aftermath right medial orbitofrontal cortex: P = .005, partial r = -0.260; DRRI-aftermath right lateral orbitofrontal cortex: P = .03, partial r = -0.196), and parahippocampal (DRRI-aftermath right: P = .03, partial r = -0.191) gyrus, as well as with higher FAT in the amygdala-hippocampus complex (DRRI-combat: P = .005, partial r = 0.254; DRRI-aftermath: P = .02, partial r = 0.223). Lower FAT in the cingulate-orbitofrontal gyri was associated with impaired response inhibition (Stroop-IS left cingulate: P < .001, partial r = -0.440; Stroop-IS right cingulate: P < .001, partial r = -0.372; Stroop-IS left medial orbitofrontal cortex: P < .001, partial r = -0.304; Stroop-IS right medial orbitofrontal cortex: P < .001, partial r = -0.340; Stroop-IN left cingulate: P < .001, partial r = -0.421; Stroop-IN right cingulate: P < .001, partial r = -0.300; Stroop-IN left medial orbitofrontal cortex: P = .01, partial r = -0.223; Stroop-IN right medial orbitofrontal cortex: P < .001, partial r = -0.343), whereas higher FAT in the mesial temporal regions was associated with improved short-term memory and processing speed (left amygdala-hippocampus complex: P < .001, partial r = -0.574; right amygdala-hippocampus complex: P < .001, partial r = 0.645; short-term memory left amygdala-hippocampus complex: P < .001, partial r = 0.570; short-term memory right amygdala-hippocampus complex: P < .001, partial r = 0.633). A history of mTBI did not modulate the association between war zone-related stress and GM diffusion.

CONCLUSIONS AND RELEVANCE

This study revealed an association between war zone-related stress and alteration of limbic GM microstructure, which was associated with cognitive functioning. These results suggest that altered limbic GM microstructure may underlie the deleterious outcomes of war zone-related stress on brain health. Military service members may benefit from early therapeutic interventions after deployment to a war zone.

Histopathological modeling of status epilepticus-induced brain damage based on in vivo diffusion tensor imaging in rats

Non-invasive magnetic resonance imaging (MRI) methods have proved useful in the diagnosis and prognosis of neurodegenerative diseases. However, the interpretation of imaging outcomes in terms of tissue pathology is still challenging. This study goes beyond the current interpretation of in vivo diffusion tensor imaging (DTI) by constructing multivariate models of quantitative tissue microstructure in status epilepticus (SE)-induced brain damage. We performed in vivo DTI and histology in rats at 79 days after SE and control animals. The analyses focused on the corpus callosum, hippocampal subfield CA3b, and layers V and VI of the parietal cortex. Comparison between control and SE rats indicated that a combination of microstructural tissue changes occurring after SE, such as cellularity, organization of myelinated axons, and/or morphology of astrocytes, affect DTI parameters. Subsequently, we constructed a multivariate regression model for explaining and predicting histological parameters based on DTI. The model revealed that DTI predicted well the organization of myelinated axons (cross-validated R = 0.876) and astrocyte processes (cross-validated R = 0.909) and possessed a predictive value for cell density (CD) (cross-validated R = 0.489). However, the morphology of astrocytes (cross-validated R &gt; 0.05) was not well predicted. The inclusion of parameters from CA3b was necessary for modeling histopathology. Moreover, the multivariate DTI model explained better histological parameters than any univariate model. In conclusion, we demonstrate that combining several analytical and statistical tools can help interpret imaging outcomes to microstructural tissue changes, opening new avenues to improve the non-invasive diagnosis and prognosis of brain tissue damage.

Analysis of homozygous and heterozygous Csf1r knockout in the rat as a model for understanding microglial function in brain development and the impacts of human CSF1R mutations

Mutations in the human CSF1R gene have been associated with dominant and recessive forms of neurodegenerative disease. Here we describe the impacts of Csf1r mutation in the rat on development of the brain. Diffusion imaging indicated small reductions in major fiber tracts that may be associated in part with ventricular enlargement. RNA-seq profiling revealed a set of 105 microglial markers depleted in all brain regions of the Csf1rko rats. There was no evidence of region or sex-specific expression of microglia-associated transcripts. Other than the microglial signature, Csf1rko had no effect on any neuronal or region-specific transcript cluster. Expression of markers of oligodendrocytes, astrocytes, dopaminergic neurons and Purkinje cells was minimally affected. However, there were defects in dendritic arborization of doublecortin-positive neurogenic precursors and expression of poly-sialylated neural cell adhesion molecule (PS-NCAM) in the dentate gyrus of the hippocampus. Heterozygous Csf1rko rats had no detectable brain phenotype. We conclude that most brain developmental processes occur normally in the absence of microglia and that CSF1R haploinsufficiency is unlikely to cause leukoencephalopathy.

Assessment of the structural complexity of diffusion MRI voxels using 3D electron microscopy in the rat brain

Validation and interpretation of diffusion magnetic resonance imaging (dMRI) requires detailed understanding of the actual microstructure restricting the diffusion of water molecules. In this study, we used serial block-face scanning electron microscopy (SBEM), a three-dimensional electron microscopy (3D-EM) technique, to image seven white and grey matter volumes in the rat brain. SBEM shows excellent contrast of cellular membranes, which are the major components restricting the diffusion of water in tissue. Additionally, we performed 3D structure tensor (3D-ST) analysis on the SBEM volumes and parameterised the resulting orientation distributions using Watson and angular central Gaussian (ACG) probability distributions as well as spherical harmonic (SH) decomposition. We analysed how these parameterisations described the underlying orientation distributions and compared their orientation and dispersion with corresponding parameters from two dMRI methods, neurite orientation dispersion and density imaging (NODDI) and constrained spherical deconvolution (CSD). Watson and ACG parameterisations and SH decomposition captured well the 3D-ST orientation distributions, but ACG and SH better represented the distributions due to its ability to model asymmetric dispersion. The dMRI parameters corresponded well with the 3D-ST parameters in the white matter volumes, but the correspondence was less evident in the more complex grey matter. SBEM imaging and 3D-ST analysis also revealed that the orientation distributions were often not axially symmetric, a property neatly captured by the ACG distribution. Overall, the ability of SBEM to image diffusion barriers in intricate detail, combined with 3D-ST analysis and parameterisation, provides a step forward toward interpreting and validating the dMRI signals in complex brain tissue microstructure.