Transgenic animal models of Alzheimer's disease and related disorders: histopathology, behavior and therapy

Alzheimer's disease (AD) is a devastating neurodegenerative disease that affects more than 15 million people worldwide. Within the next generation, these numbers will more than double. To assist in the elucidation of pathogenic mechanisms of AD and related disorders, such as frontotemporal dementia (FTDP-17), genetically modified mice, flies, fish and worms were developed, which reproduce aspects of the human histopathology, such as beta-amyloid-containing plaques and tau-containing neurofibrillary tangles (NFT). In mice, the tau pathology caused selective behavioral impairment, depending on the distribution of the tau aggregates in the brain. Beta-amyloid induced an increase in the numbers of NFT, whereas the opposite was not observed in mice. In beta-amyloid-producing transgenic mice, memory impairment was associated with increased levels of beta-amyloid. Active and passive beta-amyloid-directed immunization caused the removal of beta-amyloid plaques and restored memory functions. These findings have since been translated to human therapy. This review aims to discuss the suitability and limitations of the various animal models and their contribution to an understanding of the pathophysiology of AD and related disorders.     

Transgenic approaches to model Alzheimer's disease

Two transgenic mouse lines were generated which express human APP751 containing familial Alzheimer's disease (AD) mutations in brain neurons. These mice develop pathological features reminiscent of AD. The degree of pathology depends on both expression levels and specific mutations. In mice with more advanced pathology (APP 23), typical plaques appear at six months which increase with age and are Congo Red positive at first detection. These congophilic plaques are accompanied by neuritic changes and dystrophic cholinergic fibers. Furthermore, inflammatory processes indicated by a massive glial reaction are apparent. Most notably, plaques are immunoreactive for hyperphosphorylated tau, reminiscent of early tau pathology. A quantitative analysis of degenerative changes by state-of-the-art unbiased stereological methods revealed a significant reduction in neuronal cell bodies of the CA1 field of the hippocampus when compared to controls. This reduction is directly related to plaque load. When subjected to analysis in the Morris water maze, 18 month old APP 23 mice show a significant increase in platform finding latency throughout the entire trial when compared to non-transgenic littermates.     

Neuropsychiatric subsyndromes and brain metabolic network dysfunctions in early onset Alzheimer's disease

Neuropsychiatric symptoms (NPSs) often occur in early‐age‐of‐onset Alzheimer's disease (EOAD) and cluster into sub‐syndromes (SSy). The aim of this study was to investigate the association between ¹⁸F‐FDG‐PET regional and connectivity‐based brain metabolic dysfunctions and neuropsychiatric SSy. NPSs were assessed in 27 EOAD using the Neuropsychiatric Inventory and further clustered into four SSy (apathetic, hyperactivity, affective, and psychotic SSy). Eighty‐five percent of EOAD showed at least one NPS. Voxel‐wise correlations between SSy scores and brain glucose metabolism (assessed with ¹⁸F‐FDG positron emission tomography) were studied. Interregional correlation analysis was used to explore metabolic connectivity in the salience (aSN) and default mode networks (DMN) in a larger sample of EOAD (N = 51) and Healthy Controls (N = 57). The apathetic, hyperactivity, and affective SSy were highly prevalent (>60%) as compared to the psychotic SSy (33%). The hyperactivity SSy scores were associated with increase of glucose metabolism in frontal and limbic structures, implicated in behavioral control. A comparable positive correlation with part of the same network was found for the affective SSy scores. On the other hand, the apathetic SSy scores were negatively correlated with metabolism in the bilateral orbitofrontal and dorsolateral frontal cortex known to be involved in motivation and decision‐making processes. Consistent with these SSy regional correlations with brain metabolic dysfunction, the connectivity analysis showed increases in the aSN and decreases in the DMN. Behavioral abnormalities in EOAD are associated with specific dysfunctional changes in brain metabolic activity, in particular in the aSN that seems to play a crucial role in NPSs in EOAD. Hum Brain Mapp 37:4234–4247, 2016. © 2016 Wiley Periodicals, Inc.     

Diagnosis of early Alzheimer's disease

Current criteria for the diagnosis of Alzheimer's disease require that the patient also fulfils the criteria for dementia. This means inevitably that the disease is well-established. As treatments become available there is an important need for early diagnosis. There are two components to diagnosis; the recognition that cognitive performance is below that which would be anticipated for the individual and secondly to relate this to a specific disease process. Research studies need to identify the very earliest changes i.e. before symptoms commence. Such studies have focused on at risk populations either by virtue of age or a positive family history. Both groups have demonstrated that memory is the salient early feature, particularly verbal memory, impairment of which may precede overt symptoms by many years. The other major approach has been that of neuroimaging. Group studies of at risk individuals, for example for those who carry an apoE4 allele, have shown relative biparietal, bitemporal hypometabolism in the at risk group. Similarly, structural neuroimaging may show hippocampal atrophy at an early stage. Serial studies to determine rate of change, may be more valuable than single cross-sectional studies. Recent techniques of positional matching and registration allow precise quantitation of rates of cerebral atrophy which may be useful in early diagnosis.     

P38 MAP kinase is activated at early stages in Alzheimer's disease brain

The regional, cellular, and subcellular localization of phosphorylated p38 MAPK (pp38) was examined by immunocytochemistry, immuofluorescent multiple labeling, and immunoblotting of extracts as well as immunoprecipitates of human postmortem tissue from control and Alzheimer's disease (AD) cases at different Braak stages. "Early AD" cases (Braak stages IV-V) and a subset of Braak stage VI cases have high levels of pp38 immunoreactivity, with the most dense immunoreactivity located in CA2 and subiculum followed by CA1 in the hippocampus. On the contrary, very little pp38 was detected in age-matched controls (Braak stages 0-II). More importantly, as revealed by various multiple labeling experiments, pp38 immunoreactivity is mainly located in neurons bearing early neurofibrillary pathology, but not in typically fibrillar tangles that are densely stained by thioflavin-S. Most pp38-positive neurons only contain a small amount of phospho-tau. Additionally, pp38 immunoreactivity was not associated with senile plaques. At the subcellular level, pp38-immunoreactive granules are usually larger than the granules stained with the lysosomal marker cathepsin D. Immunoblotting with different extraction buffers and immunoprecipitation indicate that pp38 does not or only loosely binds to phospho-tau. Taken together, this study demonstrates that p38 MAPK is activated at early stages of neurofibrillary degeneration in AD hippocampus. The p38 activation may also be linked to neurodegeneration through mechanisms other than neurofibrillary tangle formation.     

Transgenic mice as a model of pre-clinical Alzheimer's disease

At diagnosis, Alzheimer's disease (AD) brains are extensively burdened with plaques and tangles and display a degree of synaptic failure most likely beyond therapeutic treatment. It is therefore crucial to identify early pathological events in the progression of the disease. While it is not currently feasible to identify and study early, pre-clinical stages of AD, transgenic (Tg) models offer a valuable tool in this regard. Here we investigated cognitive, structural and biochemical CNS alterations occurring in our newly developed McGill-Thyl-APP Tg mice (over-expressing the human amyloid precursor protein with the Swedish and Indiana mutations) prior to extracellular plaque deposition. Pre-plaque, 3-month old Tg mice already displayed cognitive deficits concomitant with reorganization of cortical cholinergic pre-synaptic terminals. Conformational specific antibodies revealed the early appearance of intracellular amyloid β (Aβ)-oligomers and fibrillar oligomers in pyramidal neurons of cerebral cortex and hippocampus. At the same age, the cortical levels of insulin degrading enzyme -a well established Aβ-peptidase, were found to be significantly down-regulated. Our results suggest that, in the McGill-Thy1-APP Tg model, functional, structural and biochemical alterations are already present in the CNS at early, pre-plaque stages of the pathology. Accumulation of intraneuronal neurotoxic Aβ-oligomers (possibly caused by a failure in the clearance machinery) is likely to be the culprit of such early, pre-plaque pathology. Similar neuronal alterations might occur prior to clinical diagnosis in AD, during a yet undefined 'latent' stage. A better understanding of such pre-clinical AD might yield novel therapeutic targets and or diagnostic tools.     

Alzheimer's disease and the aging brain

The frequencies of each of the several types of dementia are enumerated, showing that Alzheimer's disease is present in about 80% of cases. Cerebral changes associated with cognitively normal aging include shrinkage of large cortical neurons but not a significant loss of total neuronal number. Nevertheless, the population density of synapses measured by confocal microscopy does decline significantly in normal aging. The classical lesions of Alzheimer's disease are neuritic plaques and neurofibrillary tangles, and their frequency correlates with declining cognitive measures. Although amyloid is prominent in plaques, it is probably not the agent of destruction. That role seems to be held by Abeta oligomers. The strongest structural correlate with cognitive tests is synapse loss, which is probably caused by Abeta oligopeptides in the terminal axons and dendrites.     

One-carbon metabolism alteration affects brain proteome profile in a mouse model of Alzheimer's disease

Late Onset Alzheimer's Disease (LOAD) can be associated to high homocysteine level and alteration of one-carbon metabolism. We previously demonstrated in the TgCRND8 mice strain, over-expressing human amyloid-β protein precursor, that B vitamin deficiency causes alteration of one-carbon metabolism, together with unbalance of S-adenosylmethionine/S-adenosylhomocysteine levels, and is associated with AD like hallmarks as increased amyloid-β plaque deposition, hyperhomocysteinemia, and oxidative stress. The same model of nutritional deficit was used here to study the variation of the brain protein expression profile associated to B vitamin deficiency. A group of proteins mainly involved in neuronal plasticity and mitochondrial functions was identified as modulated by one-carbon metabolism. These findings are consistent with increasing data about the pivotal role of mitochondrial abnormalities in AD patho-physiology. The identified proteins might represent new potential biomarkers of LOAD to be further investigated.     

Transgenic mouse models for Alzheimer's disease: the role of GSK-3B in combined amyloid and tau-pathology

Describing and understanding the pathological processes which devastate the brain of Alzheimer's disease (AD) patients remains a major target for experimental biology. We approached this problem by generating different types of single and double transgenic mice that develop pathological hallmarks of AD. In APP-V717 mice, the progression from intracellular amyloid to diffuse and senile plaques with vascular deposits, is preceded by early defects in cognition and LTP. In Tau-P301L mice, the morbid tauopathy with intracellular filaments, cause mortality before age 1 year. Ageing APP-V717IxTau-P301L double tg mice (14-17 months) have combined AD-like pathology in hippocampus and cortex consisting of amyloid plaques and neurofibrillary tangles. Remarkably, while Tau-P301L mice die before age 1 year, the APP-V717IxTau-P301L double tg mice survive much longer, which correlates with alleviation of tauopathy in hindbrain, despite aggravation in forebrain. This hypothesis is corroborated in Tau-P301LxGSK-3B double transgenic mice, which have also an extended lifespan relative to Tau-P301L mice, that correlates with reduction of brainstem tauopathy. At the same time, Tau-P301LxGSK-3B mice have dramatic forebrain tauopathy, with "tangles in almost all neurons", although without hyper-phosphorylation of Tau. The data corroborate the hypothesis that GSK-3B is the missing link between the amyloid and tau-pathology, and position GSK-3B as prominent player in the pathogenesis in AD.     

Nonlinear EEG analysis in early Alzheimer's disease

Nonlinear EEG analysis attempts to characterize the dynamics of neural networks in the brain. Abnormalities in nonlinear EEG measures have been found repeatedly in Alzheimer's disease (AD). The present study was undertaken to investigate whether these abnormalities could already be found in the early stage of AD. In a representative sample of 49 community-dwelling elderly, Alzheimer's disease was diagnosed in 7 subjects. Correlation dimension (D2) and nonlinear prediction were measured at 16 electrodes and in two different activational states. Also, 10 surrogate data sets were generated for each EEG epoch in order to investigate the presence of nonlinear dynamics. Differences between nonlinear statistics derived from original and from surrogate data sets were expressed as Z-scores. We found lower D2 and higher predictability in the demented subjects compared to the normal subjects. The results obtained with the Z-scores pointed to changed nonlinear dynamics in frontal and temporal areas in demented subjects. However, the major differences between demented and healthy subjects are not due to nonlinearity. From this it appears that linear dynamics change first in the course of AD, followed by changes in nonlinear dynamics.     

Diffusion tensor imaging in early Alzheimer's disease

Our aim was to investigate the extent of white matter tissue damage in patients with early Alzheimer disease (AD) using diffusion tensor magnetic resonance imaging (DTI). Although AD pathology mainly affects cortical grey matter, previous magnetic resonance imaging (MRI) studies showed that changes also exist in the white matter (WM). However, the nature of AD-associated WM damage is still unclear. Conventional and DTI examinations (b=1000 s/mm(2), 25 directions) were obtained from 12 patients with early AD (Mini Mental State Examination [MMSE] score=27, Grober and Buschke test score=33.2, digit span score=5.6) and 12 sex- and age-matched volunteers. The right and left mean diffusivity (MD) and fractional anisotropy (FA) of several WM regions were pooled in each patient and control, and compared between the two groups. Volumes of the whole brain and degree of atrophy of the temporal lobe were compared between the two groups. In AD, MD was increased in the splenium of the corpus callosum and in the WM in the frontal and parietal lobes. FA was bilaterally decreased in the WM of the temporal lobe, the frontal lobe and the splenium compared with corresponding regions in controls. Values in other areas (occipital area, superior temporal area, cingulum, internal capsule, and genu of the corpus callosum) were not different between patients and controls. No correlations were found between the MMSE score and the anisotropy indices. Findings of DTI reveal abnormalities in the frontal and temporal WM in early AD patients. These changes are compatible with early temporal-to-frontal disconnections.     

Executive dysfunction in early Alzheimer's disease.

Twenty five patients with probable mild Alzheimer''s disease were assessed for deficits in executive functioning and the impact of these deficits on performance in other neuropsychological domains. The Wisconsin card sorting test, the release from proactive interference paradigm, the verbal fluency test, and the Stroop test were adopted to classify patients with (AD+) and without (AD-) executive deficits. Seven of the patients showed an impairment in executive function (AD+), defined as a performance below the cut off score in at least two of these tests. There were no significant differences in clinical assessments, demographic features, or other cognitive functions between patients. Executive dysfunction may be an early additional feature in a subgroup of patients with mild Alzheimer''s disease. Impairment on frontal lobe tests does not seem to be related to the severity or duration of disease, or to a different pattern of impairment in other cognitive domains.     

Sporadic Alzheimer's disease: the starving brain

A reduction in cerebral glucose utilization is one of the earliest signs of Alzheimer's disease. Although the exact cause of this reduction is not known, gathering evidence suggests that it is part of a complex metabolic adaptation to oxidative stress during which glycolysis and oxidative phosphorylation are turned down, glucose metabolism is shifted to the pentose phosphate pathway to generate antioxidant reducing factors such as nicotinamide adenine dinucleotide phosphate (NADPH) and glutathione, and the gamma-aminobutyric acid (GABA) shunt is activated to provide glutamate as an alternate source of energy. In the face of these adaptive metabolic changes, the Alzheimer brain runs short of energy and begins to digest itself. The very early induction of macroautophagy attests to the search for nutrients. In clinical trials, antioxidants alone have not been effectively able to influence the course of the disease as these agents do not meet the energy and nutritional requirements of the brain. Evidence is presented that gammahydroxybutyrate, a natural product of the GABA shunt, can provide the necessary energy, carbon, and antioxidant power and that its use may be able to delay the onset and progress of Alzheimer's disease.     

Regional brain trace-element studies in Alzheimer's disease

Alzheimer's disease (AD) brain trace-element imbalances in the amygdala, hippocampus and nucleus basalis of Meynert (nbM) are found in most cases to be consistent with those previously reported in samples derived principally from AD cerebral cortex (Ehmann et al., 1986). The elevation of mercury in AD nbM, as compared to age-matched controls, is the largest trace-element imbalance observed to date in AD brain. In addition to the general confirmation of imbalances for Cs, Hg, N, Na, P, and Rb noted previously in cerebral cortex samples, imbalances for Fe, K, Sc, and Zn were observed in two regions and one region also exhibited imbalances for both Co and Se. Persistent imbalances for the univalent cations Na, K, Rb and Cs support arguments for a membrane abnormality in AD. The data presented here also provide the first comprehensive simultaneous multi-element determinations in both control and AD nbM.     

Patterns of brain activity during a semantic task differentiate normal aging from early Alzheimer's disease

In a study of the effects of normal and pathological aging on semantic-related brain activity, 29 patients with Alzheimer's disease (AD) and 19 controls subjects (10 young and 9 older controls) performed a version of the Pyramids and Palm Trees Test that had been adapted for use during functional magnetic resonance imaging (fMRI). Young and older controls activated the left inferior and middle frontal gyri, precuneus and superior parietal lobule. Right frontal and left temporal cortices were activated only in the young. The AD group activated only the left prefrontal and cingulate cortex. Separate analyses of high- and low-performing AD subgroups showed a similar pattern of activation in the left frontal lobe, although activiation was more widespread in low performers. High performers significantly deactivated anterior midline frontal structures, however, while low performers did not. When the older adult and AD groups were combined, there was a significant positive correlation between left frontal and parietal activation and Mini-Mental State Examination (MMSE) score (covarying for age), suggesting a disease effect. A significant negative correlation between activation in the left temporal cortex and age (covarying for MMSE score) reflected a possible age effect. These differential effects suggest that semantic activation paradigms might aid diagnosis in those cases for whom conventional assessments lack the necessary sensitivity to detect subtle changes.     

White matter amyloid in Alzheimer's disease brain

Amyloid -protein (A) deposits in the white matter were investigated by the double immunohistochemical staining for A and neuritic, glial or vascular components. Reactive astroglia and neurite abnormality were absent around A deposits in the white matter (w-A) even those with a core. The association of w-A with blood vessels was not consistent. Aggregates of activated microglia were found to be the sole but a consistent accompaniment of A deposits even in the absence of other components such as neuron, synapse, neurite abnormality and reactive astroglia, as observed in the white matter. This suggests that the aggregates of activated microglia most likely represent one of the factors promoting the process of A deposition.