Age-related congophilic inclusions in the brains of apolipoprotein E-deficient mice

Corpora amylacea are associated with tau burden and cognitive status in Alzheimer's disease

Corpora amylacea (CA) and their murine analogs, periodic acid Schiff (PAS) granules, are age-related, carbohydrate-rich structures that serve as waste repositories for aggregated proteins, damaged cellular organelles, and other cellular debris. The structure, morphology, and suspected functions of CA in the brain imply disease relevance. Despite this, the link between CA and age-related neurodegenerative diseases, particularly Alzheimer's disease (AD), remains poorly defined. We performed a neuropathological analysis of mouse PAS granules and human CA and correlated these findings with AD progression. Increased PAS granule density was observed in symptomatic tau transgenic mice and APOE knock-in mice. Using a cohort of postmortem AD brain samples, we examined CA in cognitively normal and dementia patients across Braak stages with varying APOE status. We identified a Braak-stage dependent bimodal distribution of CA in the dentate gyrus, with CA accumulating and peaking by Braak stages II-III, then steadily declining with increasing tau burden. Refined analysis revealed an association of CA levels with both cognition and APOE status. Finally, tau was detected in whole CA present in human patient cerebrospinal fluid, highlighting CA-tau as a plausible prodromal AD biomarker. Our study connects hallmarks of the aging brain with the emergence of AD pathology and suggests that CA may act as a compensatory factor that becomes depleted with advancing tau burden.

The Accumulation of Tau-Immunoreactive Hippocampal Granules and Corpora Amylacea Implicates Reactive Glia in Tau Pathogenesis during Aging

The microtubule-associated tau protein forms pathological inclusions that accumulate in an age-dependent manner in tauopathies including Alzheimer's disease (AD). Since age is the major risk factor for AD, we examined endogenous tau species that evolve during aging in physiological and diseased conditions. In aged mouse brain, we found tau-immunoreactive clusters embedded within structures that are reminiscent of periodic acid-Schiff (PAS) granules. We showed that PAS granules harbor distinct tau species that are more prominent in 3xTg-AD mice. Epitope profiling revealed hypo-phosphorylated rather than hyper-phosphorylated tau commonly observed in tauopathies. High-resolution imaging and 3D reconstruction suggest a link between tau clusters, reactive astrocytes, and microglia, indicating that early tau accumulation may promote neuroinflammation during aging. Using postmortem human brain, we identified tau as a component of corpora amylacea (CA), age-related structures that are functionally analogous to PAS granules. Overall, our study supports neuroimmune dysfunction as a precipitating event in tau pathogenesis.

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Tau is present in mouse hippocampal granules and human corpora amylacea

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Tau accumulates with age in hippocampal granules and is accelerated in 3xTg-AD mice

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Tau immunoreactive corpora amylacea are present in Alzheimer's disease brain

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Age-related tau deposits are associated with reactive astrocytes

Corpora amylacea in human hippocampal brain tissue are intracellular bodies that exhibit a homogeneous distribution of neo-epitopes

Corpora amylacea are spherical bodies of unknown origin and function, which accumulate in the human brain during the aging process and neurodegenerative disorders. In recent work, we reported that they contain some neo-epitopes that are recognized by natural IgMs, revealing a possible link between them and the natural immune system. Here, we performed an ultrastructural study complemented with confocal microscopy in order to shed light on the formation of corpora amylacea and to precisely localize the neo-epitopes. We show that immature corpora amylacea are intracellular astrocytic structures formed by profuse cellular debris and membranous blebs entrapped in a scattered mass of randomly oriented short linear fibers. In mature corpora amylacea, the structure becomes compacted and fibrillary material constitutes the principal component. We also determined that the neo-epitopes were uniformly localized throughout the whole structure. All these observations reinforce the idea that corpora amylacea of human brain are equivalent to another type of polyglucosan bodies named PAS granules, present in mouse brain and originated from degenerative processes. All those findings support the hypothesis that corpora amylacea are involved in the entrapment of damaged materials and non-degradable products and have a role in protective or cleaning mechanisms.

Astrocytes and neurons produce distinct types of polyglucosan bodies in Lafora disease

Lafora disease (LD), the most devastating adolescence-onset epilepsy, is caused by mutations in the EPM2A or EPM2B genes, which encode the proteins laforin and malin, respectively. Loss of function of one of these proteins, which are involved in the regulation of glycogen synthesis, induces the accumulation of polyglucosan bodies (PGBs)-known as Lafora bodies (LBs) and associated with neurons-in the brain. Ageing and some neurodegenerative conditions lead to the appearance of another type of PGB called corpora amylacea, which are associated with astrocytes and contain neo-epitopes that can be recognized by natural antibodies. Here we studied the PGBs in the cerebral cortex and hippocampus of malin knockout mice, a mouse model of LD. These animals presented not only LBs associated with neurons but also a significant number of PGBs associated with astrocytes. These astrocytic PGBs were also increased in mice from senescence-accelerated mouse-prone 8 (SAMP8) strain and mice with overexpression of Protein Targeting to Glycogen (PTG^OE ), indicating that they are not exclusive of LD. The astrocytic PGBs, but not neuronal LBs, contained neo-epitopes that are recognized by natural antibodies. The astrocytic PGBs appeared predominantly in the hippocampus but were also present in some cortical brain regions, while neuronal LBs were found mainly in the brain cortex and the pyramidal layer of hippocampal regions CA2 and CA3. Our results indicate that astrocytes, contrary to current belief, are involved in the etiopathogenesis of LD.

Periodic acid-Schiff granules in the brain of aged mice: From amyloid aggregates to degenerative structures containing neo-epitopes

Brain ageing in mice leads to the progressive appearance and expansion of degenerative granular structures frequently referred as "PAS granules" because of their positive staining with periodic acid-Schiff (PAS). PAS granules are present mainly in the hippocampus, although they have also been described in other brain areas such as piriform and entorhinal cortices, and have been observed in other mammals than mice, like rats and monkeys. PAS granules have been identified as a wide range of brain deposits related to numerous neurodegenerative diseases, such as amyloid deposits, neurofibrillary tangles, Lafora bodies, corpora amylacea and polyglucosan bodies, and these identifications have generated controversy and particular theories about them. We have recently reported the presence of a neo-epitope in mice hippocampal PAS granules and the existence of natural IgM auto-antibodies directed against the neo-epitope in the plasma of the animals. The significance of the neo-epitope and the autoantibodies is discussed in this review. Moreover, we observed that the IgM anti-neo-epitope is frequently present as a contaminant in numerous commercial antibodies and is responsible of a considerable amount of false positive immunostainings, which may produce misinterpretations in the identification of the granules. Now that this point has been clarified, this article reviews and reconsiders the nature and physiopathological significance of these degenerative granules. Moreover, we suggest that neo-epitopes may turn into a useful brain-ageing biomarker and that autoimmunity could become a new focus in the study of age-related degenerative processes.

Neo-epitopes emerging in the degenerative hippocampal granules of aged mice can be recognized by natural IgM auto-antibodies

BACKGROUND

Degenerative granular structures appear progressively with age in the hippocampus of most mouse strains. We recently reported that these granules contain a neo-epitope that is recognised by IgM antibodies present as contaminants in many commercial antibodies obtained from mouse ascites and mouse or rabbit serum. We hypothesise that these anti-neo-epitope IgMs are in fact natural auto-antibodies that are generated spontaneously during the foetal stage without previous contact with external antigens and whose repertoire and reactivity pattern have been determined through evolution, being remarkably stable within species and even between species.

FINDINGS

In the present work we found that mice from the ICR-CD1, BALB/C and SAMP8 strains have anti-neo-epitope IgM antibodies in their plasma at all ages tested and even when maintained under specific opportunistic pathogen-free conditions. Moreover, we determined that these anti-neo-epitope IgMs are also present in rabbit, goat and rat serum. We also found that, in each mouse that presented hippocampal granules, the anti-neo-epitope IgMs contained in its plasma recognised the neo-epitopes in its own granules.

CONCLUSIONS

This study led to the conclusion that anti-neo-epitope IgMs are widespread natural auto-antibodies contained in the plasma of mice and other species. The presence of these natural auto-antibodies not only explains why they are frequently found as contaminants in commercial antibodies, but also paves the way for a new approach to a treatment and diagnosis of pathological brain processes based on natural IgMs and neo-epitopes.

Clustered granules present in the hippocampus of aged mice result from a degenerative process affecting astrocytes and their surrounding neuropil

Clusters of pathological granular structures appear and progressively increase in number with age in the hippocampus of several mice strains, markedly in the senescence-accelerated mouse prone 8 mice. In the present work, we performed an ultrastructural study of these granules paying special attention to the first stages of their formation, which have not been previously explored. The analysis of the immature granules allowed concluding that granules are not simple accumulations of molecular waste but the result of a degenerative process involving principally astrocytic processes, although nearby neuronal structures can be also affected. The granule generation includes the instability of the plasmatic membranes and the appearance of abnormal membranous structures that form intracellular bubbles or blebs of variable sizes and irregular shapes. These structures and some organelles degenerate producing some membranous fragments, and an assembly process of the resulting fragments generates the dense-core nucleus of the mature granule. Moreover, we found out that the neo-epitope recently described in the mature granules and localised abundantly in the membranous fragments of their dense-core nucleus emerges in the first stages of the granule formation. On the other hand, with this study, we increase the evidences that each cluster of granules is formed by the granules comprised in one astrocyte. A better knowledge of the causes of the granule formation and the function of the neo-epitope will help in both the interpretation of the physiological significance of the granules and their contribution to the degenerating processes in aging brain.

Apolipoprotein E: from lipid transport to neurobiology

Apolipoprotein (apo) E has a storied history as a lipid transport protein. The integral association between cholesterol homeostasis and lipoprotein clearance from circulation are intimately related to apoE's function as a ligand for cell-surface receptors of the low-density lipoprotein receptor family. The receptor binding properties of apoE are strongly influenced by isoform specific amino acid differences as well as the lipidation state of the protein. As understanding of apoE as a structural component of circulating plasma lipoproteins has evolved, exciting developments in neurobiology have revitalized interest in apoE. The strong and enduring correlation between the apoE4 isoform and age of onset and increased risk of Alzheimer's disease has catapulted apoE to the forefront of neurobiology. Using genetic tools generated for study of apoE lipoprotein metabolism, transgenic "knock-in" and gene-disrupted mice are now favored models for study of its role in a variety of neurodegenerative diseases. Key structural knowledge of apoE and isoform-specific differences is driving research activity designed to elucidate how a single amino acid change can manifest such profoundly significant pathological consequences. This review describes apoE through a lens of structure-based knowledge that leads to hypotheses that attempt to explain the functions of apoE and isoform-specific effects relating to disease mechanism.

Apolipoprotein E and cholesterol in aging and disease in the brain

Cholesterol can be detrimental or vital, and must be present in the right place at the right time and in the right amount. This is well known in the heart and the vascular system. However, in the CNS cholesterol is still an enigma, although several of its fundamental functions in the brain have been identified. Brain cholesterol has attracted additional attention owing to its close connection to ApoE, a key polymorphic transporter of extracellular cholesterol in humans. Indeed, both cholesterol and ApoE are so critical to fundamental activities of the brain, that the brain regulates their synthesis autonomously. Yet, similar control mechanisms of ApoE and cholesterol homeostasis may exist on either sides of the blood-brain barrier. One indication is that the APOE ε4 allele is associated with hypercholesterolemia and a proatherogenic profile on the vascular side and with increased risk of Alzheimer's disease on the CNS side. In this review, we draw attention to the association between cholesterol and ApoE in the aging and diseased brain, and to the behavior of the ApoE4 protein at the molecular level. The attempt to correlate in vivo and in vitro observations is challenging but crucial for developing future strategies to address ApoE-related aberrations in cholesterol metabolism selectively in the brain.

Proteomic identification of specific oxidized proteins in ApoE-knockout mice: relevance to Alzheimer's disease

We have examined oxidized proteins in the brain regions of wild-type (WT) and ApoE-knockout (KO) animals. Total protein oxidation in the hippocampus of young-KO (6 month) animals was approximately 2-fold greater than that of young-WT (6 month) animals and was similar to that of old-WT (18 month) and old-KO (18 month) animals. In the cortex of the same animals, the levels of total protein oxidation in all four groups were not significantly different. Two-dimensional electrophoresis (2-DE) coupled with immunostaining for protein carbonylation revealed six specific oxidation-sensitive proteins, the oxidation levels of which were increased in young-KO, old-WT, and old-KO mice compared with young-WT mice. These six oxidation-sensitive proteins were identified by mass spectrometry as glial fibrillary acidic protein, creatine kinase BB, disulfide isomerase, chaperonin subunit 5, dihydropyrimidase-related protein 2, and mortalin. These results indicate that the ApoE gene product offers protection against age-associated oxidative damage in the brain. Moreover, two of these proteins, creatine kinase and dihydropyrimidase-related protein 2, have recently been found to be oxidized in the brains of human subjects with Alzheimer's disease [Aksenov et al. J. Neurochem. 74: 2520-2527; 2000; Castegna et al. J. Neurochem. 82: 1524-1532; 2002]. These data suggest that the ApoE-knockout mouse serves as an appropriate model for studying pathogenic oxidative mechanisms influencing risk and progression of Alzheimer's disease.

Increased corticosterone secretion and early-onset of cognitive decline in female apolipoprotein E-knockout mice

In the present study, the interaction of age and apolipoprotein E (apoE)-genetic background on cognitive abilities was investigated in young (5-6 months) and aged (14-16 months) female apolipoprotein E-knockout (apoE0/0) and wild-type mice. Cognitive abilities are known to be affected by the steroid hormones corticosterone and estrogen. Therefore, we measured the activity and reactivity of the hypothalamic-pituitary-adrenal (HPA) axis expressed by circadian corticosterone concentrations and responses to novelty and controlled the regularity of the estrous cycle. Young female apoE0/0 mice acquired the water maze task and showed a similar latency and search strategy to locate the platform as young female wild-type mice. Similar corticosterone responses to novelty were observed in both genotypes. Regularity of the estrous cycle was disturbed in a small percentage of the young apoE0/0 female mice. However, in aged female apoE0/0 mice water maze performance was impaired with search strategies less persistent than in aged wild-type mice. In parallel, increased corticosterone concentrations were measured in apoE0/0 mice in response to novelty and during the circadian cycle. The percentage of mice with an irregular estrous cycle increased with age, but was comparable for apoE0/0 and wild-type mice. Thus, although disruption of the apoE gene affects the regularity of the estrous cycle in young mice, it is the enhanced corticosterone secretion, which parallels the cognitive decline in the aging female apoE0/0 mice.

Genetic loci contributing to age-related hippocampal lesions in mice

C57BL/6J mice develop genetically determined age-related hippocampal granular deposits that have some similarities to lesions seen in the brains of human patients with tau protein related neurodegenerative disorders ("tauopathies"). We sought to identify the genetic loci responsible for these in an F2 intercross of inbred mouse strains C57BL/6J and DBA/2J, using quantitative trait locus (QTL) analysis. Hippocampal lesions were shown to be PAS positive, H and E negative, and immunoreactive for tau protein and alpha synuclein, but not to Abeta 1-40 or Abeta 1-42, or for ubiquitin. These were quantitated by histomorphometry, and QTL analysis revealed a locus on chromosome 7 with a lod score of 6.5 as well as two suggestive loci on chromosome 10. The genomic data indicate that the genetic basis is complex, but with one locus playing a major role in lesion formation. These lesions may represent a useful model for investigating dysregulation of tau protein in the hippocampus.

Reversal of cognitive deficit of apolipoprotein E knockout mice after repeated exposure to a common environmental experience

This study tests the hypothesis that a history of common stressful experiences further promotes the cognitive deficit of apolipoprotein E (apoE)-knockout mice, an animal model to study aspects of Alzheimer's disease. In experiment 1, apoE-knockout and wild-type mice were repeatedly subjected to an environmental challenge (i.e. exposure to rats) and the effect was monitored on Morris water maze performance. Naive apoE-knockout mice were impaired, but surprisingly after rat stress their water maze performance improved and switched to a goal-directed search strategy. Rat stress induced in wild-type mice spatial learning deficits and an inefficient search strategy. Swim ability was not affected by rat stress and under basal conditions measures for locomotion and anxiety were similar for both genotypes. In experiments 2 and 3, we found that the rat stress paradigm attenuated the elevation of basal and stress-induced corticosterone concentrations in the apoE-knockout mice towards concentrations observed in wild-type mice. The expression of hippocampal mineralocorticoid and glucocorticoid receptor mRNA was similar in both genotypes, but in response to rat stress, the level of glucocorticoid receptor mRNA increased selectively in the CA1 pyramidal field. In conclusion, repeated exposure to a common environmental experience did abolish and reverse the difference in cognitive performance and corticosterone concentrations of apoE-knockout and wild-type mice.

Apolipoprotein E affects amyloid formation but not amyloid growth in vitro: mechanistic implications for apoE4 enhanced amyloid burden and risk for Alzheimer's disease

The transition from the partially folded soluble Abeta monomer to insoluble Abeta amyloidfibrils is seminal to the formation and growth of amyloid plaques in Alzheimer's disease (AD). A detailed understanding of the role of AD risk factors in these processes is essential to understanding the physiochemical nature of this conformational rearrangement. The apolipoprotein E epsilon4 allele, a risk factor for AD, affects AD pathology by increasing amyloid burden relative to the much more common epsilon3 allele. In the present study, in vitro models were employed to probe the effect of these proteins on kinetically distinct steps in Abeta fibrillogenesis. Formation of Abeta amyloid was faster in the presence of apoE4 than apoE3, while growth of existing plaques was unaffected by either isoform. Further, experiments with Abeta stereoisomers establish that this effect of apoE3 is mediated through interaction with oligomeric fibrillogenic intermediates rather than through specific contacts with monomeric Abeta. Consistent with the altered pathology and enhanced risk for AD associated with inheritance of the epsilon4 allele, we conclude that APOE epsilon4 is a risk factor for AD not due to a pathological gain of function of apoE4 but to a loss of protective function of apoE3.

Pathophysiology of apolipoprotein E deficiency in mice: relevance to apo E-related disorders in humans

Apolipoprotein E (apo E) deficiency (or its abnormalities in humans) is associated with a series of pathological conditions including dyslipidemia, atherosclerosis, Alzheimer's disease, and shorter life span. The purpose of this study was to characterize these conditions in apo E-deficient C57BL/6J mice and relate them to human disorders. Deletion of apo E gene in mice is associated with changes in lipoprotein metabolism [plasma total cholesterol (TC) (>+400%), HDL cholesterol (-80%), HDL/TC, and HDL/LDL ratios (-93% and -96%, respectively), esterification rate in apo B-depleted plasma (+100%), plasma triglyceride (+200%), hepatic HMG-CoA reductase activity (-50%), hepatic cholesterol content (+30%)], decreased plasma homocyst(e)ine and glucose levels, and severe atherosclerosis and cutaneous xanthomatosis. Hepatic and lipoprotein lipase activities, hepatic LDL receptor function, and organ antioxidant capacity remain unchanged. Several histological/immunohistological stainings failed to detect potential markers for neurodegenerative disease in the brain of 37-wk-old male apo E-KO mice. Apo E-KO mice may have normal growth and development, but advanced atherosclerosis and xanthomatosis may indirectly reduce their life span. Apo E plays a crucial role in regulation of lipid metabolism and atherogenesis without affecting lipase activities, endogenous antioxidant capacity, or appearance of neurodegenerative markers in 37-wk-old male mice.

Comparison of astrocytic and myocytic metabolic dysregulation in apolipoprotein E deficient and human apolipoprotein E transgenic mice

The accumulation of tubular aggregates in type II skeletal muscle fibres and fibrillo-granular inclusions in hippocampal protoplasmic astrocytes are characteristic lesions of apolipoprotein E deficient mice. Moreover these inclusions reacted immunocytochemically with an antibody specific to fragment 17-24 of the published sequence of Alzheimer's amyloid peptide. In an effort to evaluate the role of apolipoprotein E in the formation of these abnormal structures, we examined the tibialis anterior muscle and the hippocampus of several groups of animals including: (i) apolipoprotein E "knockout" mice which had been whole body irradiated with 1200 rads and bone marrow replenished with apolipoprotein E sufficient marrow; and (ii) three transgenic murine strains that had been genetically engineered to express either human apolipoprotein E2, E3 or E4 protein on an apoE deficient background. The results of this study showed that the presence of murine apolipoprotein E (even in subnormal levels in the serum) in irradiated bone marrow replenished mice and in all three (E2, E3 or E4) human apoE transgenic strains was sufficient to prevent the aggregation of sarcoplasmic tubules in the tibialis anterior type II muscle fibres. Similarly apolipoprotein E "knockout" bone marrow replenished mice and all three transgenic strains expressing the different human apolipoprotein E alleles reduced the number of the astrocytic inclusions in the hippocampus to levels not significantly different to those observed in control C57Bl6J animals. The data obtained in this study indicate that neurological and neuromuscular abnormalities found in apoE deficient mice are reversed when apoE protein is replaced in the circulation, either by bone marrow transplantation of normal apoE sufficient marrow, or by gene therapy with the apoE gene, albeit of human origin and irrespective of the allele used.

Neuroautoantibody immunoreactivity in relation to aging and stress in apolipoprotein E-deficient mice

Progressive disruption of both the neuroendocrine and immune systems has been correlated with age-associated pathogenesis in patients with Alzheimer's disease and in mice lacking apolipoprotein E (ApoE). In this study, we examined neuroautoimmune and neuroendocrine activities in relation to aging and stress in ApoE-deficient mice. An elevated level of autoantibodies against brain antigens was found in sera from ApoE-deficient mice compared to that of wild-type mice as early as 7-8 weeks of age. However, there was no significant difference between the two genotypes at this age in the effect of stress on serum corticosterone or autoantibody titers. Higher titers of autoantibodies were observed in approximately 12-week-old ApoE-deficient mice, especially in those exposed to chronic stress. Based on Western analysis, sera from ApoE-deficient mice showed a strong immunoreactivity with approximately 78 kDa and approximately 40 kDa brain abundant polypeptides, approximately 58 kDa non-brain tissue abundant antigen, and others of approximately, 80-82 kDa in both the brain and non-brain tissues. Immunofluorescence confocal microscopy showed that the major cellular components recognized by the autoimmune sera from ApoE-deficient mice were associated with neuronal cell nuclei and fiber-like structures in different regions of the brain, including the frontal cortex, lateral cortex and hippocampus. These results suggest that neuroautoimmunity associated with the aging process and exposure to chronic stress may be involved in early development of neurodegeneration in mice with ApoE-deficiency.