The aged canine (dog) is an excellent model for investigating the neurobiological changes that underlie cognitive impairment and neurodegeneration in humans, as canines and humans undergo similar pathological and behavioural changes with aging. Recent evidence indicates that a combination of environmental enrichment and antioxidant-fortified diet can be used to reduce the rate of age-dependent neuropathology and cognitive decline in aged dogs, although the mechanisms underlying these changes have not been established. We examined the hypothesis that an increase in levels of brain-derived neurotrophic factor (BDNF) is one of the factors underlying improvements in learning and memory. Old, cognitively impaired animals that did not receive any treatment showed a significant decrease in BDNF mRNA in the temporal cortex when compared with the young group. Animals receiving either an antioxidant diet or environmental enrichment displayed intermediate levels of BDNF mRNA. However, dogs receiving both an antioxidant diet and environmental enrichment showed increased levels of BDNF mRNA when compared to untreated aged dogs, approaching levels measured in young animals. BDNF receptor TrkB mRNA levels did not differ between groups. BDNF mRNA levels were positively correlated with improved cognitive performance and inversely correlated with cortical Aβ(1–42) and Aβ(1–40) levels. These findings suggest that environmental enrichment and antioxidant diet interact to maintain brain levels of BDNF, which may lead to improved cognitive performance. This is the first demonstration in a higher animal that non-pharmacological changes in lifestyle in advanced age can up-regulate BDNF to levels approaching those in the young brain.

Down syndrome (DS) is the most common genetic cause of intellectual disability in children, and the number of adults with DS reaching old age is increasing. By the age of 40 years, virtually all people with DS have sufficient neuropathology for a postmortem diagnosis of Alzheimer disease (AD). Trisomy 21 in DS leads to an overexpression of many proteins, of which at least two are involved in oxidative stress and AD: superoxide dismutase 1 (SOD1) and amyloid precursor protein (APP). In this study, we tested the hypothesis that DS brains with neuropathological hallmarks of AD have more oxidative and nitrosative stress than those with DS but without significant AD pathology, as compared with similarly aged-matched non-DS controls. The frontal cortex was examined in 70 autopsy cases (n=29 control and n=41 DS). By ELISA, we quantified soluble and insoluble Aβ40 and Aβ42, as well as oligomers. Oxidative and nitrosative stress levels (protein carbonyls, HNE-bound proteins, and 3-nitrotyrosine) were measured by slot-blot. We found that soluble and insoluble Aβ and oligomers increase as a function of age in DS frontal cortex. Of the oxidative stress markers, HNE-bound proteins were increased overall in DS. Protein carbonyls were correlated with Aβ40 levels. These results suggest that oxidative damage, but not nitrosative stress, may contribute to the onset and progression of AD pathogenesis in DS. Conceivably, treatment with antioxidants may provide a point of intervention to slow pathological alterations in DS.

The aged canine is a higher animal model that naturally accumulates β-amyloid (Aβ) and shows age-related cognitive decline. However, profiles of Aβ accumulation in different species (40 vs. 42), its assembly states, and Aβ precursor protein (APP) processing as a function of age remain unexplored. In this study, we show that Aβ increases progressively with age as detected in extracellular plaques and biochemically extractable Aβ40 and Aβ42 species. Soluble oligomeric forms of the peptide, with specific increases in an Aβ oligomer migrating at 56kDa, also increase with age. Changes in APP processing could potentially explain why Aβ accumulates, and we show age-related shifts towards decreased total APP protein and non-amyloidogenic (α-secretase) processing coupled with increased amyloidogenic (β-secretase) cleavage of APP. Importantly, we describe Aβ pathology in the cingulate and temporal cortex and provide a description of oligomeric Aβ across the canine lifespan. Our findings are in line with observations in the human brain, suggesting that canines are a valuable higher animal model for the study of Aβ pathogenesis.

Intersectin 1 (ITSN1) is a chromosome 21 (HSA21) gene product encoding a multi-domain scaffold protein that functions in endocytosis, signal transduction and is implicated in Down Syndrome, Alzheimer’s Disease, and potentially other neurodegenerative diseases through activation of c-Jun N-terminal kinase (JNK). We report for the first time that ITSN1 proteins are elevated in Down Syndrome individuals of varying ages. However, ITSN1 levels decreased in aged Down Syndrome cases with Alzheimer’s Disease-like neuropathology. Analysis of a novel ITSN1 transgenic mouse reveals that ITSN1 overexpression results in a sex-dependent decrease in locomotor activity. This study reveals a link between overexpression of specific ITSN1 isoforms and behavioral phenotypes and has implications for human neurodegenerative diseases such as Down Syndrome and Alzheimer’s Disease.

Aged canines naturally accumulate several types of neuropathology that may have links to cognitive decline. On a gross level, significant cortical atrophy occurs with age along with an increase in ventricular volume based on magnetic resonance imaging studies. Microscopically, there is evidence of select neuron loss and reduced neurogenesis in the hippocampus of aged dogs, an area critical for intact learning and memory. The cause of neuronal loss and dysfunction may be related to the progressive accumulation of toxic proteins, oxidative damage, cerebrovascular pathology, and changes in gene expression. For example, aged dogs naturally accumulate human-type beta-amyloid peptide, a protein critically involved with the development of Alzheimer’s disease in humans. Further, oxidative damage to proteins, DNA/RNA and lipids occurs with age in dogs. Although less well explored in the aged canine brain, neuron loss, and cerebrovascular pathology observed with age are similar to human brain aging and may also be linked to cognitive decline. Interestingly, the prefrontal cortex appears to be particularly vulnerable early in the aging process in dogs and this may be reflected in dysfunction in specific cognitive domains with age.

Statins have been suggested to protect against Alzheimer’s disease (AD). Recently, however, we reported that aged dogs that underwent chronic statin treatment exhibited cognitive deficits compared with age matched controls. In human studies, blood levels of Coenzyme Q10 (CoQ10) decrease with statin use. CoQ10 is important for proper mitochondrial function and is a powerful antioxidant, two important factors for cognitive health in aging. Thus, the current study tested the hypothesis that CoQ10 levels in the serum and/or parietal cortex are decreased in statin treated dogs and are associated with poorer cognition. Six aged beagles (>8 years) were administered 80 mg/day of atorvastatin for 14.5 months and compared with placebo-treated animals. As predicted, serum CoQ10 was significantly lower in statin-treated dogs. Parietal cortex CoQ10 was not different between the two groups. However, poorer cognition was correlated with lower parietal cortex CoQ10. This study in dogs suggests that serum CoQ10 is reduced with atorvastatin treatment. CoQ10 levels in brain may linked to impaired cognition in response to atorvastatin, in agreement with previous reports that statins may have a negative impact on cognition in the elderly.

Individuals with Down syndrome over age 40 years are at risk for developing dementia of the Alzheimer type and have evidence for chronic oxidative stress. There is a paucity of treatment trials for dementia in Down syndrome in comparison to Alzheimer disease in the general (non-Down syndrome) population. This 2-year randomized, double-blind, placebo-controlled trial assessed whether daily oral antioxidant supplementation (900 IU of alpha-tocopherol, 200 mg of ascorbic acid and 600 mg of alpha-lipoic acid) was effective, safe and tolerable for 53 individuals with Down syndrome and dementia. The outcome measures comprised a battery of neuropsychological assessments administered at baseline and every 6 months. Compared to the placebo group, those individuals receiving the antioxidant supplement showed neither an improvement in cognitive functioning nor a stabilization of cognitive decline. Mean plasma levels of alpha-tocopherol increased ~2-fold in the treatment group and were consistently higher than the placebo group over the treatment period. Pill counts indicated good compliance with the regimen. No serious adverse events attributed to the treatment were noted. We conclude that antioxidant supplementation is safe, though ineffective as a treatment for dementia in individuals with Down syndrome and Alzheimer type dementia. Our findings are similar to studies of antioxidant supplementation in Alzheimer disease in the general population. The feasibility of carrying out a clinical trial for dementia in Down syndrome is demonstrated.

Background

The apolipoprotein E (APOE) ε2 allele has been suggested as having a protective effect and delaying the age at onset of Alzheimer disease.

Objective

To describe a dissociation between findings neuropathologic with normal cognition in a woman with severe Alzheimer disease with the APOE ε2/ε2 genotype.

Design

Case report from a community based prospective study of persons 90 years or older (The 90+ Study).

Participant

A 92-year-old woman without dementia with the APOE ε2/ε2 genotype who lived independently without significant cognitive or functional loss and was a participant in The 90+ Study. She died in December 2004, and postmortem examination of her brain was performed.

Intervention

Neurologic examination and a battery of neuropsychological tests were performed 6 months and 1 month before death. Neuropathologic examination included Braak and Braak staging for senile plaques and neurofibrillary tangles.

Results

Neuropathologic examination of the brain revealed advanced senile plaque and neurofibrillary tangle disease consistent with a high likelihood of Alzheimer disease. At clinical evaluation, the participant demonstrated no dementia and only mild cognitive deficits.

Conclusions

The APOE genotype may have contributed to maintenance of cognition despite advanced neuropathologic findings of Alzheimer disease. This case suggests that the APOE ε2 isoform may have a protective effect against cognitive decline in Alzheimer disease that may be independent from senile plaques and neurofibrillary tangles.

Aged dogs and humans share complex cognitive and pathological responses to aging. Specifically, dogs develop Alzheimer’s Disease (AD) like beta-amyloid (Aβ) that are associated with cognitive deficits. Currently, therapeutic approaches to prevent AD are targeted towards reduced production, aggregation and increased clearance of Aβ. The current review discusses cognition and neuropathology of the aging canine model and how it has and continues to be useful in further understanding the safety and efficacy of potential AD prevention therapies targeting Aβ.

Over the past 15 years, insights into sterol metabolism have improved our understanding of the relationship between lipids and common conditions such as atherosclerosis and Alzheimer's Disease (AD). A better understanding of sterol lipid metabolism in individuals with Down Syndrome (DS) may help elucidate how this population's unique metabolic characteristics influence their risks for atherosclerosis and AD. To revisit the question of whether sterol lipid parameters may be altered in DS subjects, we performed a pilot study to assess traditional serum sterol lipids and lipoproteins, as well as markers of sterol biosynthesis, metabolites, and plant sterols in 20 subjects with DS compared to age-matched controls. Here we report that the levels of nearly all lipids and lipoproteins examined are similar to control subjects, suggesting that trisomy 21 does not lead to pronounced general alterations in sterol lipid metabolism. However, the levels of serum brassicasterol were markedly reduced in DS subjects.

We examined the relationships between normal aging, Alzheimer’s disease (AD), and brain levels of sex steroid hormones in men and women. In postmortem brain tissue from neuropathologically normal, postmenopausal women, we found no age-related changes in brain levels of either androgens or estrogens. In comparing women with and without AD at different ages, brain levels of estrogens and androgens were lower in AD cases aged 80 years and older but not significantly different in the 60–79 year age range. In male brains, we observed that normal aging was associated with significant decreases in androgens but not estrogens. Further, in men aged 60–79 years, brain levels of testosterone but not estrogens were lower in cases with mild neuropathological changes as well as those with advanced AD neuropathology. In male cases over age 80, brain levels hormones did not significantly vary by neuropathological status. To begin investigating the relationships between hormone levels and indices of AD neuropathology, we measured brain levels of soluble β-amyloid (Aβ). In male cases with mild neuropathological changes, we found an inverse relationship between brain levels of testosterone and soluble Aβ. Collectively, these findings demonstrate sex-specific relationships between normal, age-related depletion of androgens and estrogens in men and women, which may be relevant to development of AD.

Alzheimer disease (AD) is an age-related neurodegenerative disorder characterized by progressive memory loss, inability to perform the activities of daily living and personality changes. Unfortunately, drugs effective for this disease are limited to acetylcholinesterase inhibitors that do not impact disease pathogenesis. Statins, which belong to the class of cholesterol-reducing drugs, were proposed as novel agents useful in AD therapy, but the mechanism underlying their neuroprotective effect is still unknown. In this study, we show that atorvastatin may have antioxidant effects, in aged beagles, that represent a natural higher mammalian model of AD. Atorvastatin (80 mg/day for 14.5 months) significantly reduced lipoperoxidation, protein oxidation and nitration, and increased GSH levels in parietal cortex of aged beagles. This effect was specific for brain because it was not paralleled by a concomitant reduction in all these parameters in serum. In addition, atorvastatin slightly reduced the formation of cholesterol oxidation products in cortex but increased the 7-ketocholesterol/total cholesterol ratio in serum. We also found that increased oxidative damage in the parietal cortex was associated with poorer learning (visual discrimination task). Thus, a novel pharmacological effect of atorvastatin mediated by reducing oxidative damage may be one mechanism underlying benefits of this drug in AD.

Adults with Down syndrome (DS) are at risk for developing Alzheimer disease (AD). While plasma Aβ is known to be elevated in DS, its relationship to cognitive functioning is unknown. To assess this relationship, samples from two groups of subjects were used. In the first group, nondemented adults with DS were compared to: 1) a group of young and old individuals without DS and 2) to a group of patients with AD. Compared to these controls, there were significantly higher levels of plasma Aβ in nondemented adults with DS while AD patients showed lower levels of plasma Aβ. A larger second group included demented and nondemented adults with DS, in order to test the hypothesis that plasma Aβ may vary as a function of dementia and ApoE genotype. Plasma Aβ levels alone did not dissociate DS adults with and without dementia. However, in demented adults with DS, ApoE4 was associated with higher Aβ40 but not Aβ42. After controlling for level of intellectual disability (mild, moderate, profound) and the presence or absence of dementia, there was an improved prediction of neuropsychological scores by plasma Aβ. In summary, plasma Aβ can help predict cognitive function in adults with DS independently of the presence or absence of dementia.

The molecular bases of Alzheimer's disease (AD) remain unclear. We used a lipidomic approach to identify lipid abnormalities in the brains of subjects with AD (N = 37) compared to age-matched controls (N = 17). The analyses revealed statistically detectable elevations in levels of non-esterified monounsaturated fatty acids (MUFAs) and mead acid (20:3n-9) in mid-frontal cortex, temporal cortex and hippocampus of AD patients. Further studies showed that brain mRNAs encoding for isoforms of the rate-limiting enzyme in MUFAs biosynthesis, stearoyl-CoA desaturase (SCD-1, SCD-5a and SCD-5b), were elevated in subjects with AD. The monounsaturated/saturated fatty acid ratio (‘desaturation index’) – displayed a strong negative correlation with measures of cognition: the Mini Mental State Examination test (r = −0.80; P = 0.0001) and the Boston Naming test (r = −0.57; P = 0.0071). Our results reveal a previously unrecognized role for the lipogenic enzyme SCD in AD.

Age-related cerebrovascular dysfunction contributes to ischemic stroke, intracerebral hemorrhages, microbleeds, cerebral amyloid angiopathy (CAA), and cognitive decline. Importantly, there is increasing recognition that this dysfunction plays a critical aging secondary role in many neurodegenerative diseases, including Alzheimer’s disease (AD). Atherosclerosis, hypertension, and CAA are the most common causes of blood brain barrier (BBB) lesions. The accumulation of amyloid beta (Aβ) in the cerebrovascular system is a significant risk factor for intracerebral hemorrhage (ICH), and has been linked to endothelial transport failure and blockage of perivascular drainage. Moreover, recent anti-Aβ immunotherapy clinical trials demonstrated efficient clearance of parenchymal amyloid deposits, but have been plagued by CAA-associated adverse events. While management of hypertension and atherosclerosis can reduce the incidence of ICH, there are currently no approved therapies for attenuating CAA. Thus, there is a critical need for new strategies that improve BBB function and limit the development of beta-amyloidosis in the cerebral vasculature.

Human studies are reviewed concerning whether “aging”-related mechanisms contribute to Alzheimer’s disease (AD) pathogenesis. AD is defined by specific neuropathology: neuritic amyloid plaques and neocortical neurofibrillary tangles. AD pathology is driven by genetic factors related not to aging per se, but instead to the amyloid precursor protein (APP). In contrast to genes involved in APP-related mechanisms, there is no firm connection between genes implicated in human “accelerated aging” diseases (progerias) and AD. The epidemiology of AD in advanced age is highly relevant but deceptively challenging to address given the low autopsy rates in most countries. In extreme old age, brain diseases other than AD approximate AD prevalence while the impact of AD pathology appears to peak by age 95 and decline thereafter. Many distinct brain diseases other than AD afflict older human brains and contribute to cognitive impairment. Additional prevalent pathologies include cerebrovascular disease and hippocampal sclerosis, both high-morbidity brain diseases that appear to peak in incidence later than AD chronologically. Because of these common brain diseases of extreme old age, the epidemiology differs between clinical “dementia” and the subset of dementia cases with AD pathology. Additional aging-associated mechanisms for cognitive decline such as diabetes and synapse loss have been linked to AD and these hypotheses are discussed. Criteria are proposed to define an “aging-linked” disease, and AD fails all of these criteria. In conclusion, it may be most fruitful to focus attention on specific pathways involved in AD rather than attributing it to an inevitable consequence of aging.

Previously we showed that anti-Aβ peptide immunotherapy significantly attenuated Alzheimer’s-like amyloid deposition in the central nervous system of aged canines. In this report we have characterized the changes that occurred in the humoral immune response over 2.4 years in canines immunized repeatedly with aggregated Aβ1–42 (AN1792) formulated in alum adjuvant. We observed a rapid and robust induction of anti-Aβ antibody titers, which were associated with an anti-inflammatory T helper type 2 (Th2) response. The initial antibody response was against dominant linear epitope at the N-terminus region of the Aβ1–42 peptide, which is identical to the one in humans and vervet monkeys. After multiple immunizations the antibody response drifted toward the elevation of antibodies that recognized conformational epitopes of assembled forms of Aβ and other types of amyloid. Our findings indicate that prolonged immunization results in distinctive temporal changes in antibody profiles, which may be important for other experimental and clinical settings.

Studies suggest that frontotemporal lobar degeneration with transactive response DNA-binding protein of 43 kDa (TDP-43) proteinopathy (FTLD-TDP) is heterogeneous with division into four or five subtypes. To determine the degree of heterogeneity and the validity of the subtypes, we studied neuropathological variation within the frontal and temporal lobes of 94 cases of FTLD-TDP using quantitative estimates of density and principal components analysis (PCA). A PCA based on the density of TDP-43 immunoreactive neuronal cytoplasmic inclusions, oligodendroglial inclusions, neuronal intranuclear inclusions, and dystrophic neurites, surviving neurons, enlarged neurons, and vacuolation suggested that cases were not segregated into distinct subtypes. Variation in the density of the vacuoles was the greatest source of variation between cases. A PCA based on TDP-43 pathology alone suggested that cases of FTLD-TDP with progranulin (GRN) mutation segregated to some degree. The pathological phenotype of all four subtypes overlapped but subtypes 1 and 4 were the most distinctive. Cases with coexisting motor neuron disease (MND) or hippocampal sclerosis (HS) also appeared to segregate to some extent. We suggest: (1) pathological variation in FTLD-TDP is best described as a ‘continuum’ without clearly distinct subtypes, (2) vacuolation was the single greatest source of variation and reflects the ‘stage’ of the disease, and (3) within the FTLD-TDP ‘continuum’ cases with GRN mutation and with coexisting MND or HS may have a more distinctive pathology.

Background

Age-related neurodegenerative diseases share a number of important pathological features, such as accumulation of misfolded proteins as amyloid oligomers and fibrils. Recent evidence suggests that soluble amyloid oligomers and not the insoluble amyloid fibrils may represent the primary pathological species of protein aggregates.

Results

We have produced several monoclonal antibodies that specifically recognize prefibrillar oligomers and do not recognize amyloid fibrils, monomer or natively folded proteins. Like the polyclonal antisera, the individual monoclonals recognize generic epitopes that do not depend on a specific linear amino acid sequence, but they display distinct preferences for different subsets of prefibrillar oligomers. Immunological analysis of a number of different prefibrillar Aβ oligomer preparations show that structural polymorphisms exist in Aβ prefibrillar oligomers that can be distinguished on the basis of their reactivity with monoclonal antibodies. Western blot analysis demonstrates that the conformers defined by the monoclonal antibodies have distinct size distributions, indicating that oligomer structure varies with size. The different conformational types of Aβ prefibrillar oligomers can serve as they serve as templates for monomer addition, indicating that they seed the conversion of Aβ monomer into more prefibrillar oligomers of the same type.

Conclusions

These results indicate that distinct structural variants or conformers of prefibrillar Aβ oligomers exist that are capable of seeding their own replication. These conformers may be analogous to different strains of prions.

Previous work has shown that a diet enriched with antioxidants and mitochondrial co-factors improves cognition in aged dogs, which was accompanied by a reduction oxidative damage in the brain. The objective of the present study was to assess the effects of supplementation with mitochondrial co-factors on cognition and plasma protein carbonyl levels in aged dogs. Specifically, we aimed to test whether the individual or combined action of lipoic acid (LA) and acetyl-L-carnitine (ALCAR) could account for the beneficial effects of the enriched diet that contained both plus antioxidants. Dogs were given LA or ALCAR, alone and then in combination and cognition was assessed using a spatial learning task and two discrimination and reversal paradigms. Dogs receiving the ALCAR supplement showed an increase in protein carbonyl levels that was associated with increased error scores on the spatial task, and which was reduced upon additional supplementation with LA. We did not observe significant positive effects on cognition. The present findings suggest that short-term supplementation with LA and ALCAR is insufficient to improve cognition in aged dogs, and that the beneficial effects of the full spectrum diet arose from either the cellular antioxidants alone or their interaction with LA and ALCAR.

Reduced brain levels of docosahexaenoic acid (C22:6n-3), a neurotrophic and neuroprotective fatty acid, may contribute to cognitive decline in Alzheimer's disease. Here, we investigated whether the liver enzyme system that provides docosahexaenoic acid to the brain is dysfunctional in this disease. Docosahexaenoic acid levels were reduced in temporal cortex, mid-frontal cortex and cerebellum of subjects with Alzheimer's disease, compared to control subjects (P = 0.007). Mini Mental State Examination (MMSE) scores positively correlated with docosahexaenoic/α-linolenic ratios in temporal cortex (P = 0.005) and mid-frontal cortex (P = 0.018), but not cerebellum. Similarly, liver docosahexaenoic acid content was lower in Alzheimer's disease patients than control subjects (P = 0.011). Liver docosahexaenoic/α-linolenic ratios correlated positively with MMSE scores (r = 0.78; P<0.0001), and negatively with global deterioration scale grades (P = 0.013). Docosahexaenoic acid precursors, including tetracosahexaenoic acid (C24:6n-3), were elevated in liver of Alzheimer's disease patients (P = 0.041), whereas expression of peroxisomal d-bifunctional protein, which catalyzes the conversion of tetracosahexaenoic acid into docosahexaenoic acid, was reduced (P = 0.048). Other genes involved in docosahexaenoic acid metabolism were not affected. The results indicate that a deficit in d-bifunctional protein activity impairs docosahexaenoic acid biosynthesis in liver of Alzheimer's disease patients, lessening the flux of this neuroprotective fatty acid to the brain.

Recent evidence has suggested a role for soluble oligomeric Aβ species in the pathology of Alzheime’s disease (AD). Fibrillar plaque deposits are present in non-demented individuals and levels of soluble Aβ correlate better with cognitive dysfunction in AD and transgenic mouse models. We have previously reported that there are at least two conformationally distinct types of Aβ oligomers: prefibrillar oligomers that are kinetic intermediates in fibril assembly reactions and are specifically recognized by A11 antibody and fibrillar oligomers that may represent fibril seeds or small pieces of fibrils and are recognized by a fibril specific antibody, OC. We have examined the levels of these two types of oligomers in the PBS soluble fraction of brain tissue from control cases, cases with senile degenerative changes (SDC) and AD patients. We found that the levels of soluble fibrillar oligomers detected by OC antibody are significantly elevated in multiple brain regions of AD patients. The elevated fibrillar oligomer levels were found not to be an artifact of tissue homogenization, nor a result of increased Aβ or APP levels. The concentration of fibrillar oligomers in adjacent brain regions of the same patient can vary widely and were not detected in post mortem cerebrospinal fluid. In contrast, the level of prefibrillar oligomers are variable in both AD and age matched controls, indicating that they are not correlated with cognitive dysfunction and suggesting that they precede dementia in AD. Significant correlations were found between the levels of fibrillar oligomers and cognitive decline (MMSE scores) as well as the neuropathological hallmarks of AD. These results indicate that fibrillar oligomers may play a key role in the pathology of AD and may be a new target for diagnostic and therapeutic development.

Studies in humans suggest that lifestyle factors can have a beneficial impact on the risk for developing cognitive decline and dementia with age. There is growing evidence that maintaining a physically and intellectually active lifestyle can positively impact cognitive ability in older individuals. Dietary factors, such as the intake of antioxidants, may also prevent age-related cognitive decline. However, studies in humans are challenging; many variables cannot be controlled, making it difficult for researchers to determine the exact types and quantities of enrichment and dietary factors necessary for positive effects on cognition. Studies in animal models of human aging allow researchers to precisely control such variables, and can be used to assess the mechanisms and molecular pathways underlying any positive effects. Here we review the results of an intervention study using a canine model of human aging. The study was unique in that it compared the effects of dietary antioxidant supplementation alone and in combination with behavioral enrichment. We found that both interventions lead to improvements in cognitive ability in aged dogs; however, combining the treatments preserved cognition to a greater extent than either treatment alone. Overall, the results suggest that antioxidant supplementation and behavioral enrichment target separate yet complementary molecular pathways to improve cognition, and support the idea that combinations of treatments to improve cognition and slow brain aging will produce greater benefits than single interventions.

The study of Alzheimer’s disease (AD) pathogenesis requires the use of animal models that develop some amount of amyloid pathology in the brain. Aged canines (beagles) naturally accumulate human-type amyloid β-peptide (Aβ) and develop parallel declines in cognitive function. However, the type and quantity of biochemically extracted Aβ in brain and cerebrospinal fluid (CSF), its link to aging and similarity to human aging, has not been examined systematically. Thirty beagles, aged 4.5–15.7 years, were studied. Aβ40 and Aβ42 were measured in CSF by ELISA, and from SDS and formic acid extracted prefrontal cortex. A sample of the contralateral hemisphere, used to assess immunohistochemical amyloid load, was used for comparison. In the brain, increases in Aβ42 were detected at a younger age, prior to increases in Aβ40, and were correlated with an increased amyloid load. In the CSF, Aβ42 decreased with age while Aβ40 levels remained constant. The CSF Aβ42/40 ratio was also a good predictor of the amount of Aβ in the brain. The amount of soluble oligomers in CSF was inversely related to brain extractable Aβ, whereas oligomers in the brain were correlated with SDS soluble Aβ42. These findings indicate that the Aβ in the brain of the aged canine exhibits patterns that mirror Aβ deposited in the human brain. These parallels support the idea that the aged canine is a useful intermediate between transgenic mice and humans for studying the development of amyloid pathology, and is a potentially useful model for the refinement of therapeutic interventions.

A recent study demonstrated the lack of beta-amyloid (Aβ) plaque formation and accumulation of the amyloid precursor protein (APP) in a triple transgenic mouse model of Alzheimer’s disease (3×Tg-AD) following overexpression of the anti-apoptotic protein, Bcl-2 (Rohn et al., J. Neurosci. 28: 3051-9, 2008). The supposition from that study was the accumulation of APP resulted from a decrease in caspase-mediated processing of APP. To determine a direct role for the caspase-cleavage of APP in 3×Tg-AD mice, we designed a site-directed caspase-cleavage antibody to APP and demonstrated it is a specific marker for caspase-cleaved APP. Application of this antibody revealed neuronal staining in the hippocampus and subiculum of 3×Tg-AD mice. These results were confirmed utilizing a similar site-directed antibody to caspase-cleaved APP (APPneo). The caspase cleavage of APP as well as the formation of extracellular Aβ plaques was prevented in 3×Tg-AD animals overexpressing Bcl-2. These results provide further support that caspases play a proximal role in promoting the pathology associated with AD.