Magnetic resonance T2 relaxation time at 7 Tesla associated with amyloid β pathology and age in a double-transgenic mouse model of Alzheimer’s disease

Neuroimaging of Mouse Models of Alzheimer’s Disease

Magnetic resonance imaging (MRI) and positron emission tomography (PET) have made great strides in the diagnosis and our understanding of Alzheimer’s Disease (AD). Despite the knowledge gained from human studies, mouse models have and continue to play an important role in deciphering the cellular and molecular evolution of AD. MRI and PET are now being increasingly used to investigate neuroimaging features in mouse models and provide the basis for rapid translation to the clinical setting. Here, we provide an overview of the human MRI and PET imaging landscape as a prelude to an in-depth review of preclinical imaging in mice. A broad range of mouse models recapitulate certain aspects of the human AD, but no single model simulates the human disease spectrum. We focused on the two of the most popular mouse models, the 3xTg-AD and the 5xFAD models, and we summarized all known published MRI and PET imaging data, including contrasting findings. The goal of this review is to provide the reader with broad framework to guide future studies in existing and future mouse models of AD. We also highlight aspects of MRI and PET imaging that could be improved to increase rigor and reproducibility in future imaging studies.

Magnetic Resonance Imaging in Animal Models of Alzheimer's Disease Amyloidosis

Amyloid-beta (Aβ) plays an important role in the pathogenesis of Alzheimer's disease. Aberrant Aβ accumulation induces neuroinflammation, cerebrovascular alterations, and synaptic deficits, leading to cognitive impairment. Animal models recapitulating the Aβ pathology, such as transgenic, knock-in mouse and rat models, have facilitated the understanding of disease mechanisms and the development of therapeutics targeting Aβ. There is a rapid advance in high-field MRI in small animals. Versatile high-field magnetic resonance imaging (MRI) sequences, such as diffusion tensor imaging, arterial spin labeling, resting-state functional MRI, anatomical MRI, and MR spectroscopy, as well as contrast agents, have been developed for preclinical imaging in animal models. These tools have enabled high-resolution in vivo structural, functional, and molecular readouts with a whole-brain field of view. MRI has been used to visualize non-invasively the Aβ deposits, synaptic deficits, regional brain atrophy, impairment in white matter integrity, functional connectivity, and cerebrovascular and glymphatic system in animal models of Alzheimer's disease amyloidosis. Many of the readouts are translational toward clinical MRI applications in patients with Alzheimer's disease. In this review, we summarize the recent advances in MRI for visualizing the pathophysiology in amyloidosis animal models. We discuss the outstanding challenges in brain imaging using MRI in small animals and propose future outlook in visualizing Aβ-related alterations in the brains of animal models.

Progression of Alzheimer's disease and effect of scFv-h3D6 immunotherapy in the 3xTg-AD mouse model: An in vivo longitudinal study using Magnetic Resonance Imaging and Spectroscopy

Alzheimer's disease (AD) is an incurable disease that affects most of the 47 million people estimated as living with dementia worldwide. The main histopathological hallmarks of AD are extracellular β-amyloid (Aβ) plaques and intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein. In recent years, Aβ-immunotherapy has been revealed as a potential tool in AD treatment. One strategy consists of using single-chain variable fragments (scFvs), which avoids the fragment crystallizable (Fc) effects that are supposed to trigger a microglial response, leading to microhemorrhages and vasogenic edemas, as evidenced in clinical trials with bapineuzumab. The scFv-h3D6 generated by our research group derives from this monoclonal antibody, which targets the N-terminal of the Aβ peptide and recognizes monomers, oligomers and fibrils. In this study, 3xTg-AD mice were intraperitoneally and monthly treated with 100 μg of scFv-h3D6 (a dose of ~3.3 mg/kg) or PBS, from 5 to 12 months of age (-mo), the age at which the mice were sacrificed and samples collected for histological and biochemical analyses. During treatments, four monitoring sessions using magnetic resonance imaging and spectroscopy (MRI/MRS) were performed at 5, 7, 9, and 12 months of age. MRI/MRS techniques are widely used in both human and mouse research, allowing to draw an in vivo picture of concrete aspects of the pathology in a non-invasive manner and allowing to monitor its development across time. Compared with the genetic background, 3xTg-AD mice presented a smaller volume in almost all cerebral regions and ages examined, an increase in both the intra and extracellular Aβ_1-42 at 12-mo, and an inflammation process at this age, in both the hippocampus (IL-6 and mIns) and cortex (IL-6). In addition, treatment with scFv-h3D6 partially recovered the values in brain volume, and Aβ, IL-6, and mIns concentrations, among others, encouraging further studies with this antibody fragment.

Dorsal hippocampal changes in T2 relaxation times are associated with early spatial cognitive deficits in 5XFAD mice

T2 relaxation time (T2) alterations may serve as markers for early detection and disease progression monitoring by reflecting brain microstructural integrity in Alzheimer's disease (AD). However, the characteristics of T2 alterations during the early stage of AD remain elusive. We explored T2 alterations and their possible correlations with cognitive function in 5XFAD mice at early ages (1, 2, 3, and 5 months of age). Voxel-based analysis (VBA) and region of interest (ROI) analysis showed a decreased T2 in the hippocampus of 2-, 3-, and 5-month-old 5XFAD mice compared to those of controls. The dorsal hippocampal T2 decreased earlier than the ventral hippocampus T2. A significant correlation was observed between Morris water maze (MWM) test cognitive behavior and the dorsal hippocampus T2 in 5XFAD mice. These results indicated that the microstructural integrity of brain tissues, particularly the hippocampus, was impaired early and the impairment became more extensive and severe during disease progression. Furthermore, the dorsal hippocampus is a crucial component involved in spatial cognition impairment in young 5XFAD mice.

Comparison of the effects of nimodipine and deferoxamine on brain injury in rat with subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) may lead to brain atrophy and cognitive dysfunction. This study aimed to compare the efficacy of nimodipine and deferoxamine on these sequelae of SAH. A rat model of SAH was established by the double-hemorrhage method. These rats were injected with saline (intraperitoneal, IP), nimodipine (IP), or deferoxamine (IP and intranasal) every 12 h for 5 days after SAH. The MRI scanning, including magnetic resonance angiography, diffusion tensor imaging, T_2-weighted imaging, was performed to detect the brain structure. The levels of iron metabolism-related proteins were examined by Western blot analysis. The Morris water maze (MWM) test was used to assess the cognitive function. Then, then neurons in the cortex and hippocampus were counted on hematoxylin and eosin-stained brain sections. Significant cerebral vasospasm (CVS) was found in the saline and deferoxamine groups, but not in the nimodipine group. Cerebral peduncle injury was detected in the saline and nimodipine groups, but not significantly in the deferoxamine group. Compared with nimodipine, deferoxamine reduced transferrin (Tf), Tf receptor, and ferritin levels after SAH. The MWM performances were significantly worse in the saline and nimodipine groups than that in the deferoxamine group. Brain atrophy and neuronal losses were more significant in the saline and nimodipine groups than in the deferoxamine group. Nimodipine significantly ameliorated CVS, but it did not improve the late changes in brain structure and cognitive function. Deferoxamine effectively reduced neuronal cell death and ameliorated cognitive function after SAH.

Association between delayed recall and T2* relaxation time of the subiculum in adolescents: Implications for ultra-high-field magnetic resonance imaging

AIM

The aim of this study was to assess neuropsychological correlations with the T2* relaxation time (T2*-RT) of hippocampal subregions in adolescents using ultra-high-field (UHF) 7.0-T magnetic resonance imaging (MRI).

METHODS

We assessed the T2*-RT of hippocampal subregions in 31 healthy 11th- or 12th-grade high school students using an UHF 7.0-T MRI system. T2*-RT of the cornu ammonis (CA) 1, CA2, CA3, and CA4 subregions and the subiculum were calculated for both the left and right hippocampus. Seven subtests of the Cambridge Neuropsychological Test Automated Battery were administered to the subjects to assess visuospatial memory.

RESULTS

Poor performances in delayed recall in the pattern-recognition test were significantly correlated with longer T2*-RT in the bilateral subiculum (right, r = -0.480, P = 0.006; left, r = -0.648, P < 0.001) and the left CA2 (r = -0.480, P = 0.006).

CONCLUSION

This study showed that longer T2*-RT in the subiculum were associated with poorer performances in delayed recall in the visual memory tasks. This finding suggests that the subiculum might play a predominant role in delayed recall in adolescents.

Multifaceted assessment of the APP/PS1 mouse model for Alzheimer's disease: Applying MRS, DTI, and ASL

Transgenic animal models of Alzheimer's disease (AD) can mimic pathological and behavioral changes occurring in AD patients, and are usually viewed as the first choice for testing novel therapeutics. Validated biomarkers, particularly non-invasive ones, are urgently needed for AD diagnosis or evaluation of treatment results. However, there are few studies that systematically characterize pathological changes in AD animal models. Here, we investigated the brain of 8-month-old amyloid precursor protein/presenilin 1 (APP/PS1) transgenic and wild-type (WT) mice, employing 7.0-T magnetic resonance imaging (MRI). Magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), and arterial spin labeling (ASL) were obtained through micro-MRI scanning. After MRI examination in both transgenic (n = 12) and WT (n = 12) mice, immunohistochemical staining and ultrastructural analysis were subsequently performed. Cerebral blood flow (CBF) was significantly decreased in the left hippocampus, left thalamus, and right cortex of AD mice (P < 0.05). Moreover, MRS showed significantly changed NAA/Cr, Glu/Cr, and mI/Cr ratios in the hippocampus of transgenic mice. While only NAA/Cr and mI/Cr ratios varied significantly in the cortex of transgenic mice. Regarding DTI imaging, however, the values of FA, MD, DA and DR were not significantly different between transgenic and WT mice. Finally, it is worth noting that pathological damage of metabolism, CBF, and white matter was more distinct between transgenic and WT mice by pathological examination. Altogether, our results suggest that intravital imaging evaluation of 8-month-old APP/PS1 transgenic mice by MRS and ASL is an alternative tool for AD research.

Magnetic resonance imaging relaxation time in Alzheimer's disease

The magnetic resonance imaging (MRI) relaxation time constants, T1 and T2, are sensitive to changes in brain tissue microstructure integrity. Quantitative T1 and T2 relaxation times have been proposed to serve as non-invasive biomarkers of Alzheimer's disease (AD), in which alterations are believed to not only reflect AD-related neuropathology but also cognitive impairment. In this review, we summarize the applications and key findings of MRI techniques in the context of both AD subjects and AD transgenic mouse models. Furthermore, the possible mechanisms of relaxation time alterations in AD will be discussed. Future studies could focus on relaxation time alterations in the early stage of AD, and longitudinal studies are needed to further explore relaxation time alterations during disease progression.

MRI in the Study of Animal Models of Neurodegenerative Diseases

Magnetic Resonance Imaging (MRI) is an important tool to study various animal models of degenerative diseases. This chapter describes routine protocols of T ₁-, T ₂-, and T ₂*-weighted and diffusion-weighted MRI for rodent brain and spinal cord. These protocols can be used to measure atrophy, axonal and myelin injury and changes in white matter connectivity.

Transverse relaxation of cerebrospinal fluid depends on glucose concentration

PURPOSE

To evaluate the biophysical processes that generate specific T₂ values and their relationship to specific cerebrospinal fluid (CSF) content.

MATERIALS AND METHODS

CSF T_2s were measured ex vivo (14.1T) from isolated CSF collected from human, rat and non-human primate. CSF T_2s were also measured in vivo at different field strength in human (3 and 7T) and rodent (1, 4.7, 9,4 and 11.7T) using different pulse sequences. Then, relaxivities of CSF constituents were measured, in vitro, to determine the major molecule responsible for shortening CSF T₂ (2s) compared to saline T₂ (3s). The impact of this major molecule on CSF T₂ was then validated in rodent, in vivo, by the simultaneous measurement of the major molecule concentration and CSF T₂.

RESULTS

Ex vivo CSF T₂ was about 2.0s at 14.1T for all species. In vivo human CSF T₂ approached ex vivo values at 3T (2.0s) but was significantly shorter at 7T (0.9s). In vivo rodent CSF T₂ decreased with increasing magnetic field and T₂ values similar to the in vitro ones were reached at 1T (1.6s). Glucose had the largest contribution of shortening CSF T₂in vitro. This result was validated in rodent in vivo, showing that an acute change in CSF glucose by infusion of glucose into the blood, can be monitored via changes in CSF T₂ values.

CONCLUSION

This study opens the possibility of monitoring glucose regulation of CSF at the resolution of MRI by quantitating T₂.