Emerging evidence suggests that Alzheimer's disease (AD) disproportionately affect women in both occurrence and severity. However, little is known about the biological mechanisms underlying these sex differences. There are evidences that changes in metabolism may be present earlier than structural brain changes during AD development. In this study, we present a systematic longitudinal in vivo MRI study in combination with in vivo spectroscopy to investigate the sex-related structural and metabolic differences in various brain regions of AD mouse. In vivo metabolic profile and anatomical differences were monitored longitudinally in same male and female AβPPswe, PSEN1dE9 and AβPPswe transgenic (tg) mice and wt littermates using MRI and novel localized two dimensional MR spectroscopy at 9.4T. At each time point histological analysis were performed in separate male and female mice of same age group. To investigate significant age, sex and tg related differences, a mixed model analysis was performed. Our results show difference in metabolic profile in male and female AD mice including apparent decline in glucose, excitatory neurotransmitter glutamate as well as decrease in neuronal marker N-acetylaspartate (NAA) in cortex and hippocampus. Histological comparisons show that some of these metabolic changes appear earlier than the pathological differences and may have mechanistic role in sex-specific differences in AD development. Our results provide evidence that metabolic changes during AD development are clearly influenced by sex. These results are in line with the known higher risk of AD in women.
Better knowledge of the distribution of iron in the brains of Alzheimer’s disease (AD) patients may facilitate the development of an in vivo magnetic resonance (MR) marker for AD and may cast light on the role of this potentially toxic molecule in the pathogenesis of AD. Several histological iron staining techniques have been used in the past but they have not been systematically tested for sensitivity and specificity. This article compares three histochemical techniques and ferritin immunohistochemistry to visualize iron in paraffin-embedded human AD brain tissue. The specificity of the histochemical techniques was tested by staining sections after iron extraction. Iron was demonstrated in the white matter, in layers IV/V of the frontal neocortex, in iron containing plaques, and in microglia. In our hands, these structures were best visualized using the Meguro iron stain, a method that has not been described for iron staining in human brain or AD in particular. Ferritin immunohistochemistry stained microglia and iron containing plaques similar to the Meguro method but was less intense in myelin-associated iron. The Meguro method is most suitable for identifying iron-positive structures in paraffin-embedded human AD brain tissue.
Abnormal iron distribution in the isocortex is increasingly recognized as an in vivo marker for Alzheimer's disease (AD). However, the contribution of iron accumulation to the AD pathology is still poorly understood. In this study, we investigated: 1) frontal cortical iron distribution in AD and normal aging and 2) the relation between iron distribution and degree of AD pathology. We used formalin fixed paraffin embedded frontal cortex from 10 AD patients, 10 elder, 10 middle aged, and 10 young controls and visualized iron with a modified Perl's histochemical procedure. AD and elderly subjects were not different with respect to age and sex distribution. Iron distribution in the frontal cortex was not affected by normal aging but was clearly different between AD and controls. AD showed accumulation of iron in plaques, activated microglia, and, in the most severe cases, in the mid-cortical layers along myelinated fibers. The degree of altered iron accumulations was correlated to the amount of amyloid-β plaques and tau pathology in the same block, as well as to Braak stage (p < 0.001). AD and normal aging show different iron and myelin distribution in frontal cortex. These changes appear to occur after the development of the AD pathological hallmarks. These findings may help the interpretation of high resolution in vivo MRI and suggest the potential of using changes in iron-based MRI contrast to indirectly determine the degree of AD pathology in the frontal cortex.
Initial studies regarding the co-localization of Aβ deposits and MRI hypointensities in ex vivo human brain tissue using MR microscopy showed conflicting results.1,2 Recently, Aβ deposits were convincingly coregistered with MR hypointensities by direct imaging of a single histological section.3 In addition, scans on a 7T whole body clinical system using large ex vivo brain samples showed hypointensities which correlated with the presence of Alzheimer's disease (AD), even though direct histological conformation was not feasible.4 This current study further investigates the effects of Aβ deposits on MRI signal intensity as a function of spatial resolution, thereby aiming to explore whether MR microscopy findings are translatable to a clinical setting. Four millimeter thick samples of the neocortex were obtained from eight neuropathologically confirmed AD and four non-demented control formalin-fixed brains. Using a Bruker 9.4T scanner, T2*-weighted multiple gradient echo images were acquired at 40,100, 200 and 400 μm isotropic resolution. Following paraffin embedding, 8 μm sections were immunostained for Aβ (DAKO), digitized and matched with their corresponding MR images. Only those brains with Aβ deposits showed spatially-corresponding small hypointensities in the 40 μm scan. The number of hypointensities increased linearly with the number of deposits seen on histology (Fig1A-C). Detection rate improved with longer TE, suggesting a T2* effect as described previously.1,3,4 Only brains with high loads of Aβ deposits showed similar hypointensities at the lowest resolutions (Fig.1A), which were independently verified not to correspond to any vasculature which can result in similar effects (Fig.1B). The histological section suggests these hypointensities correlate to areas of clustered Aβ deposits. These results confirm earlier observations that Aβ deposits within human brains can be detected by high resolution T2*-weighted imaging. While direct correlation with histology is difficult due to partial volume effects, our approach allowed us to verify the source of hypointensities even at the lowest resolution. These initial findings thereby strongly suggest that Aβ deposits within AD brains can be visualized with MR even at clinically-achievable resolutions depending on their spatial distribution. 1Benveniste PNAS 1999; 2Dhenain NMR Biomed 2002; 3Meadowcroft JMRI 2009; 4van Rooden Radiology 2009
This thesis adresses a variety of early markers of
Alzheimer's disease, using MRI, histology and MRS. MRS is found to be
promising for early diagnosis of AD. However this study is done on mice and
should be replicated on AD patients over time. Besides the early markers the
thesis descibes a potential difference between male and female in the
development of AD in the brain.
To explore the ability of whole-body 7.0-T magnetic resonance (MR) imaging to depict differences in aspects of the cerebral cortex of postmortem human brain specimens with cerebral amyloid beta deposition in connection with Alzheimer disease (AD), Down syndrome, or sporadic or hereditary cerebral amyloid angiopathy (CAA) and control brain specimens lacking such deposition.This study was approved by the local institutional review board. In all cases, informed consent was obtained to perform autopsy and to use the tissues for research purposes. T2- and T2*-weighted MR imaging was performed in formalin-fixed samples of brain tissue from six subjects with AD changes, seven with CAA, and five subjects without immunohistochemical evidence of cerebral amyloid beta deposition. All MR images were visually assessed for hypointense foci in and inhomogeneity of the cortex. Sensitivity, specificity, and kappa values of these MR imaging features in the detection of histologic changes were calculated.High-spatial-resolution 0.3 x 0.3 x 0.3-mm three-dimensional T2*-weighted images revealed hypointense foci, inhomogeneity of the cortex, or both in all specimens with brain amyloid beta deposition. These MR imaging features were observed in none of the control specimens.The finding of postmortem susceptibility-weighted changes in the cerebral cortex of patients with cerebral amyloidosis with a human 7.0-T MR imaging system opens up the possibility of obtaining in vivo radiologic evidence of cerebral amyloid beta deposition.
For the interpretation of magnetic resonance imaging (MRI) abnormalities in brain pathology, often ex vivo tissue is used. The purpose of this study was to determine the pathological substrate of several distinct forms of MR hypointensities that were found in formalin-fixed brain tissue with amyloid-beta deposits. Samples of brain cortex were scanned using effective transverse relaxation time-weighted protocols at several resolutions on a 9.4 T MRI scanner. High resolution MRI showed large coarse hypointensities throughout the cortical gray and white matter, corresponding to macroscopic discolorations and microscopic circumscribed areas of granular basophilic neuropil changes, without any further specific tissue reactions or amyloid-beta related pathology. These coarse MRI hypointensities were identified as localized areas of absent neuropil replaced by membrane/myelin sheath remnants using electron microscopy. Interestingly, the presence/absence of these tissue alterations was not related to amyloid deposits, but strongly correlated to the fixation time of the samples in unrefreshed formalin. These findings show that prolonged storaged of formalin fixed brain tissue results in subtle histology artifacts, which show on MRI as hypointensities that on first appearance are indistinguishable from genuine brain pathology. This indicates that postmortem MRI should be interpreted with caution, especially if the history of tissue preservation is not fully known.
Epidemiological studies indicate that the incidence of Alzheimer's disease (AD) is higher in women than in men. There is evidence that changes in metabolites in the brain associated with the development of AD are present earlier than structural brain
