Marc Dhenain

In vivo MRI and histological evaluation of brain atrophy in APP/PS1 transgenic mice

Regional cerebral atrophy was evaluated in APP/PS1 mice harboring mutated transgenes linked to familial Alzheimers disease, using complementary methods. In vivo high resolution MRI was selected for measurements of brain atrophy and associated cerebrospinal fluid dilation; histological analysis was performed to reveal localized atrophies and to evaluate amyloid burden. Young APP/PS1 mice examined at a pre-amyloid stage (10 weeks) showed disruption in development (reduced intracranial and brain volumes). Comparison of young and old (24 months) mice, indicated that both APP/PS1 and control brains endure growth during adulthood. Aged APP/PS1 animals showed a moderate although significant global brain atrophy and a dilation of CSF space in posterior brain regions. The locus of this atrophy was identified in the midbrain area and not, as expected, at isocortical/hippocampal levels. Atrophy was also detected in fiber tracts. The severity of brain atrophy in old APP/PS1 mice was not correlated with the extent of cerebral amyloidosis. The relevance of current transgenic mouse models for the study of brain atrophy related to Alzheimers disease is discussed.

Age-related evolution of amyloid burden, iron load, and MR relaxation times in a transgenic mouse model of Alzheimer's disease

T1 and T2 magnetic resonance relaxation times have the potential to provide biomarkers of amyloid-beta deposition that could be helpful to the development of new therapies for Alzheimers disease. Here, we measured T1 and T2 times as well as plaques and iron loads in APP/PS1 mice, which model brain amyloidosis, and control PS1 mice. Iron was mostly associated with amyloid deposits in APP/PS1 animals, while it was diffuse in the PS1 mice. T1 was negatively correlated with age in most structures in APP/PS1 animals. This may be related to the age-associated myelin loss described in APP/PS1 mice rather than to amyloid deposition. T2 in the subiculum of adult APP/PS1 animals was lower than in PS1 mice, which may be related to the very high amyloid and iron loads in this region. T2 in the subiculum could thus serve as an early marker of the amyloid pathology.

Characterization of in vivo MRI detectable thalamic amyloid plaques from APP/PS1 mice

Amyloid deposits are one of the hallmarks of Alzheimers disease. Recent studies, in transgenic mice modeling Alzheimers disease showed that, using in vivo, contrast agent-free, MRI, thalamic amyloid plaques are more easily detected than other plaques of the brain. Our study evaluated the characteristics of these thalamic plaques in a large population of APP/PS1, PS1 and C57BL/6 mice. Thalamic spots were detected in all mice but with different frequency and magnitude. Hence, the prevalence and size of the lesions were higher in APP/PS1 mice. However, even in APP/PS1 mice, thalamic spots did not occur in all the old animals. In APP/PS1 mice, spots detection was related to high iron and calcium load within amyloid plaques and thus reflects the ability of such plaque to capture large amounts of minerals. Interestingly, calcium and iron was also detected in extra-thalamic plaques but with a lower intensity. Hypointense lesions in the thalamus were not associated with the iron load in the tissue surrounding the plaques, nor with micro-hemorrhages, inflammation, or a neurodegenerative context.

Passive staining: A novel ex vivo MRI protocol to detect amyloid deposits in mouse models of Alzheimer's disease

Amyloid plaques are one of the hallmarks of Alzheimers disease (AD). This study evaluated a novel μMRI strategy based on “passive staining” of brain samples by gadoteric acid. The protocol was tested at 4.7T on control animals and APP/PS1 mice modeling AD lesions. T1 was strongly decreased in passively stained brains. On high‐resolution 3D gradient echo images, the contrast between the cortex and subcortical structures was highly improved due to a T2* effect. The brains of APP/PS1 mice revealed plaques as hypointense spots. They appeared larger in long compared to short TE images. This suggests that, after passive staining, plaques caused a susceptibility effect. This easily performed protocol is a complementary method to classic histology to detect the 3D location of plaques. It may also be used for the validation of in vivo MRI protocols for plaque detection by facilitating registration with histology via post mortem MRI. Magn Reson Med, 2006.

Regional atrophy in the brain of lissencephalic mouse lemur primates: Measurement by automatic histogram-based segmentation of MR images

Age‐related regional cerebral atrophy was evaluated in a small lissencephalic primate, model of cerebral aging. Twelve mouse lemurs (Microcebus murinus), ages 1.9–10.9 years (maximum life span: 12 years), were studied. 3D inversion‐recovery fast spin‐echo MR images (isotropic resolution = 234 μm) were recorded at 4.7 T with a surface coil actively decoupled from the transmitting birdcage probe. The surface coil‐related sensitivity gradient was corrected by normalization with images from an agar and NaCl phantom. An automatic statistical segmentation technique based on a classification‐maximization algorithm was tested on digital phantoms that mimicked the brain and applied to the 3D brain images. Segmented 3D maps that displayed gray matter, white matter, and cerebro‐spinal fluid (CSF) voxels were computed. The ventricles and peri‐encephalic spaces were categorized into 14 regions, defined on brain atlases on the basis of cytoarchitectural and anatomical criteria. The volume of CSF voxels belonging to each of these regions was calculated as an index of regional atrophy. Dilation of the mammillary fossa was an early event in the aging process. CSF accumulation within the occipital, parieto‐temporal, temporal, and frontal ventral peri‐encephalic spaces was particularly marked in the oldest animals that also displayed ophthalmologic alterations. Magn Reson Med 50:984–992, 2003.

IC-P1-019: Patterns of cerebral atrophy and neuropathological alterations in mouse lemur primates suggest an evolutive disease

Background: Cerebral atrophy is a common pattern in Alzheimer’s disease and in many other neurodegenerative processes (1). It is also reported in aged mouse lemur primates (2). These small animals (12 cm, maximal life span of 12 years) can also display diffuse amyloid depositions and tau pathology while aging (3). Methods: The location of cerebral atrophy was assessed in 31 mouse lemurs aged from 1.9 to 11.3 years. 3D MR images (isotropic resolution 234 m) were recorded on a 4.7T Bruker system. An automatic segmentation technique was used to detect cerebro-spinal fluid (CSF) voxels in areas surrounding the cortex (2). Then, CSF voxels surrounding various functional regions (frontal and parietal (FPcort), parieto-temporal (PTcort), temporal (Tcort), occipital (Ocort) cortices) were manually labeled and counted. Histological evaluation (amyloid (anti-A 4G8 antibodies (Biovalley, France)) and GFAP stainings) was also performed in 7 lemurs (6.4 to 11.2 years) previously evaluated by MRI. Results: Significant CSF increase in pericortical regions was detected in only one young animal. The prevalence of cerebral atrophy increased with age (25% and 75% in middle aged and old animals), following a characteristic, sequential, pattern. First, some animals (n 3) displayed CSF accumulation in regions surrounding the FPcort (stage 1). In five animals, in addition to FPcort, the atrophy process involved one cortical region adjacent to FPcort. In 4 animals, this additional region was PTcort (stage 2a) while it was Tcort for one animal (stage 2b). In 2 additional animals, the atrophy process involved FPcort, PTcort and Tcort (stage 3). Finally, in 5 animals the atrophy involved FPcort, PTcort, Tcort, and Ocort. Only two of these lemurs had extracellular amyloid deposits. Most of these severely atrophied animals had intracellular amyloid deposits in cortical and hippocampal regions and activated glial accumulation. Conclusions: Our data suggest that cerebral atrophy in mouse lemurs follows a sequential pathway and that neuropathological alterations other than extracellular amyloid deposition should be investigated to explain the brain atrophy. Acknowledgements: Longevity-GIS, ‘NeuroscienceACI-programs’ (French Minister for Research), France-Alzheimer-Association.

Amyloid deposits imaging by passive staining in mouse models of Alzheimer’s disease

A 1-42). Conclusions: Compact neuritic A plaques were abundant in the frontal cortex and absent in the cerebellum. 6-CN-BTA-1 bound to blood vessels and A 1-16/tau dual-labeled compact neuritic plaques in the frontal cortex; binding was not detected in the cerebellum. 6-CN-BTA-1 was more readily detected in A 1-40-immunoreactive (ir) plaques than in A 1-42-ir plaques. Diffuse non-neuritic A deposits were devoid of 6-CN-BTA-1 staining. Pretreatment of tissue sections with formic acid completely abolished 6-CN-BTA-1 staining. Our results demonstrate that 6-CN-BTA-1 labels preferentially compact neuritic plaques in AD brain, suggesting its affinity for binding to aggregated A . These data aid in interpreting the binding properties of the parent compound of 6-CN-BTA-1, PIB, which is currently under evaluation as an in vivo diagnostic marker of diseasespecific A plaque pathology progression in AD brains.