Existence of different types of senile plaques between brain and spinal cord of TgCRND8 mice

Established Beta Amyloid Pathology Is Unaffected by TREM2 Elevation in Reactive Microglia in an Alzheimer's Disease Mouse Model

Several genetic studies have identified a rare variant of triggering receptor expressed on myeloid cells 2 (TREM2) as a risk factor for Alzheimer's disease (AD). However, findings on the effects of TREM2 on Aβ deposition are quite inconsistent in animal studies, requiring further investigation. In this study, we investigated whether elevation of TREM2 mitigates Aβ pathology in TgCRND8 mice. We found that peripheral nerve injury resulted in a robust elevation of TREM2 exclusively in reactive microglia in the ipsilateral spinal cord of aged TgCRND8 mice at the age of 20 months. TREM2 expression appeared on day 1 post-injury and the upregulation was maintained for at least 28 days. Compared to the contralateral side, neither amyloid beta plaque load nor soluble Aβ40 and Aβ42 levels were attenuated upon TREM2 induction. We further showed direct evidence that TREM2 elevation in reactive microglia did not affect amyloid-β pathology in plaque-bearing TgCRND8 mice by applying anti-TREM2 neutralizing antibody to selectively block TREM2. Our results question the ability of TREM2 to ameliorate established Aβ pathology, discouraging future development of disease-modifying pharmacological treatments targeting TREM2 in the late stage of AD.

Intracisternal injection of beta-amyloid seeds promotes cerebral amyloid angiopathy

Beta amyloid (Aβ) is a key component of parenchymal Aβ plaques and vascular Aβ fibrils, which lead to cerebral amyloid angiopathy (CAA) in Alzheimer's disease (AD). Recent studies have revealed that Aβ contained in the cerebrospinal fluid (CSF) can re-enter into brain through paravascular spaces. However, whether Aβ in CSF may act as a constant source of pathogenic Aβ in AD is still unclear. This study aimed to examine whether Aβ pathology could be worsened when CSF Aβ level was enhanced by intra-cisternal infusion of aged brain extract containing abundant Aβ in TgCRND8 host mice. TgCRND8 mouse is an AD animal model which develops predominant parenchymal Aβ plaques in the brain at as early as 3 months of age. Here, we showed that single intracisternal injection of Aβ seeds into TgCRND8 mice before the presence of Aβ pathology induced robust prion-like propagation of CAA within 90 days. The induced CAA is mainly distributed in the cerebral cortex, hippocampus and thalamus of TgCRND8 mice. Surprisingly, despite the robust increase in CAA levels, the TgCRND8 mice had a marked decrease in parenchymal Aβ plaques and the plaques related neuroinflammation in the brains compared with the control mice. These results amply indicate that Aβ in CSF may act as a source of Aβ contributing to the growth of vascular Aβ deposits in CAA. Our findings provide experimental evidence to unravel the mechanisms of CAA formation and the potential of targeting CSF Aβ for CAA.

The Parietal Atrophy Score on Brain Magnetic Resonance Imaging is a Reliable Visual Scale

Aims

The purpose of the study was to evaluate the reliability of our new visual scale for a quick atrophy assessment of parietal lobes on brain Magnetic Resonance Imaging (MRI) among different professionals. A good agreement would justify its use for differential diagnosis of neurodegenerative dementias, especially early-onset Alzheimer’s Disease (AD), in clinical settings.

Methods

The visual scale named the Parietal Atrophy Score (PAS) is based on a semi-quantitative assessment ranging from 0 (no atrophy) to 2 (prominent atrophy) in three parietal structures (sulcus cingularis posterior, precuneus, parietal gyri) on T1-weighted MRI coronal slices through the whole parietal lobes. We used kappa statistics to evaluate intra-rater and inter-rater agreement among four raters who independently scored parietal atrophy using PAS. Rater 1 was a neuroanatomist (JM), rater 2 was an expert in MRI acquisition and analysis (II), rater 3 was a medical student (OP) and rater 4 was a neurologist (DS) who evaluated parietal atrophy twice in a 3-month interval to assess intra-rater agreement. All raters evaluated the same 50 parietal lobes on brain MRI of 25 cognitively normal individuals with even distribution across all atrophy degrees from none to prominent according to the neurologist’s rating.

Results

Intra-rater agreement was almost perfect with the kappa value of 0.90. Inter-rater agreement was moderate to substantial with kappa values ranging from 0.43-0.86.

Conclusion

The Parietal Atrophy Score is the reliable visual scale among raters of different professions for a quick evaluation of parietal lobes on brain MRI within 1-2 minutes. We believe it could be used as an adjunct measure in differential diagnosis of dementias, especially early-onset AD.

Association Between Axonopathy and Amyloid Plaques in the Spinal Cord of the Transgenic Mice of Alzheimer's Disease

Axonopathy manifested by axon swellings might constitute one of the earliest pathological features of Alzheimer's disease. It has been proposed that axonopathy might be associated with the origin of Aβ plaques. However, how axonopathy leads to Aβ plaque pathogenesis remains elusive. Our previous studies have shown that Aβ neuropathology (mainly diffuse plaques) selectively occurred in the regions of corticospinal tract (CST) pathway and its innervated region in the spinal cord of TgCRND8 mice. In this study, we investigated the occurrence and progression of axonopathy and the possible implication in Aβ plaque pathogenesis in the spinal cord of TgCRND8 mice. By anterograde labeling of CST system with a neuroanatomical tracer, we found that dilated corticospinal axons started to appear at 7 months, then exhibited an age-dependent increase. These abnormal structures appear before any plaque deposits are visible in the spinal cord of the mice. Importantly, they colocalized with Aβ plaques in either the white matter or gray matter of the spinal cord at later stages, suggesting that these axonal swellings might represent the initial stages of Aβ plaque formation, and could play a role in Aβ plaque pathogenesis. Furthermore, using ultrastructural analysis we demonstrated that intracellular contents in the axonal dystrophies such as various dense vesicles leaked out into the extracellular matrix under a condition of axon swelling rupture in CST pathways of spinal cord. This provided precise structural evidence that how the Aβ leaks out from the axonal dystrophies into extracellular matrix and how an axonal swelling might serve as a nidus of amyloid plaque formation.

Motor deficits are independent of axonopathy in an Alzheimer's disease mouse model of TgCRND8 mice

There have been an increasing number of reports of non-cognitive symptoms in Alzheimer's disease (AD). Some symptoms are associated with the loss of motor functions, e.g. gait disturbances, disturbed activity level and balance. Consistent with clinical findings, several AD mouse models harboring amyloid pathology develop motor impairment. Although the factors that contribute to the motor deficits have not yet been determined, it has been suggested that axonopathy is one of the key factors that may contribute to this particular feature of the disease. Our previous study found that TgCRND8 mice exhibited profound motor deficits as early as 3 months old. In this study, we explored the possible factors that may be related to motor deficits in TgCRND8 mice. Results from silver, neurofilament and amyloid precursor protein (APP) staining revealed no axonopathy occurred in the brain and spinal cord of TgCRND8 mice at the age of 3 months. Anterograde labeling of corticospinal tract of spinal cord and electronic microscopy (EM) analysis showed that no axonopathy occurred in TgCRND8 mice at the age of 3 months. According to these results, it could be concluded that no axonal alterations were evident in the TgCRND8 mice when motor deficits was overt. Thus, axonopathy may play a less prominent role in motor deficits in AD. These results suggest that mechanisms by which motor function undergo impairment in AD need to be further studied.