Cerebral preamyloid deposits and congophilic angiopathy in aged dogs

3D-printed collagen/silk fibroin/secretome derived from bFGF-pretreated HUCMSCs scaffolds enhanced therapeutic ability in canines traumatic brain injury model

The regeneration of brain tissue poses a great challenge because of the limited self-regenerative capabilities of neurons after traumatic brain injury (TBI). For this purpose, 3D-printed collagen/silk fibroin/secretome derived from human umbilical cord blood mesenchymal stem cells (HUCMSCs) pretreated with bFGF scaffolds (3D-CS-bFGF-ST) at a low temperature were prepared in this study. From an in vitro perspective, 3D-CS-bFGF-ST showed good biodegradation, appropriate mechanical properties, and good biocompatibility. In regard to vivo, during the tissue remodelling processes of TBI, the regeneration of brain tissues was obviously faster in the 3D-CS-bFGF-ST group than in the other two groups (3D-printed collagen/silk fibroin/secretome derived from human umbilical cord blood mesenchymal stem cells (3D-CS-ST) group and TBI group) by motor assay, histological analysis, and immunofluorescence assay. Satisfactory regeneration was achieved in the two 3D-printed scaffold-based groups at 6 months postsurgery, while the 3D-CS-bFGF-ST group showed a better outcome than the 3D-CS-ST group.

Animal models of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA), due to vascular amyloid β (Aβ) deposition, is a risk factor for intracerebral haemorrhage and dementia. CAA can occur in sporadic or rare hereditary forms, and is almost invariably associated with Alzheimer's disease (AD). Experimental (animal) models are of great interest in studying mechanisms and potential treatments for CAA. Naturally occurring animal models of CAA exist, including cats, dogs and non-human primates, which can be used for longitudinal studies. However, due to ethical considerations and low throughput of these models, other animal models are more favourable for research. In the past two decades, a variety of transgenic mouse models expressing the human Aβ precursor protein (APP) has been developed. Many of these mouse models develop CAA in addition to senile plaques, whereas some of these models were generated specifically to study CAA. In addition, other animal models make use of a second stimulus, such as hypoperfusion or hyperhomocysteinemia (HHcy), to accelerate CAA. In this manuscript, we provide a comprehensive review of existing animal models for CAA, which can aid in understanding the pathophysiology of CAA and explore the response to potential therapies.

Ageing changes in cat brains demonstrated by β-amyloid and AT8-immunoreactive phosphorylated tau deposits

The life expectancy of domestic pet cats is increasing, along with the occurrence of geriatric-onset behavioural problems, such as cognitive dysfunction syndrome (CDS). While the cause of CDS is unclear, it has been suggested that it may result from age-related neurodegeneration. In aged and in particular senile human beings, histopathological changes may include the extracellular accumulation of plaque-like deposits of β-amyloid (Aβ) protein and the intracellular accumulation of an abnormally hyperphosphorylated form of the microtubule-associated protein, tau. In severe cases, the latter may form into neurofibrillary tangles. Brain material was assessed from 19 cats, aged from 16 weeks to 14 years; 17 of which had clinical signs of neurological dysfunction. Immunohistochemical methods were used to detect Aβ and its intracellular precursor protein (amyloid precursor protein (APP)) and hyperphosphorylated-tau.

APP was constitutively expressed, with diffuse staining of neurons and blood vessels being detected in all cats. More intense staining and diffuse extracellular Aβ staining deposits were found within the deep cortical areas of the anterior- and occasionally mid-cerebrum of seven cats, all of which were over 10 years of age. Neurons staining intensely positive for AT8-immunoreactivity were seen in two cats, aged 11 and 13 years. However, no mature neurofibrillary tangles were detected. This study demonstrated that extracellular Aβ accumulation and AT8-immunoreactivity within neurons are age-related phenomena in cats, and that they can occur concurrently. There are similarities between these changes and those observed in the brains of aged people and other old mammals.

Tau hyperphosphorylation in synaptosomes and neuroinflammation are associated with canine cognitive impairment

Canine cognitive impairment syndrome (CDS) represents a group of symptoms related to the aging of the canine brain. These changes ultimately lead to a decline of memory function and learning abilities, alteration of social interaction, impairment of normal housetraining, and changes in sleep-wake cycle and general activity. We have clinically examined 215 dogs, 28 of which underwent autopsy. With canine brains, we performed extensive analysis of pathological abnormalities characteristic of human Alzheimer's disease and frontotemporal lobar degeneration, including β-amyloid senile plaques, tau neurofibrillary tangles, and fused in sarcoma (FUS) and TAR DNA-binding protein 43 (TDP43) inclusions. Most demented dogs displayed senile plaques, mainly in the frontal and temporal cortex. Tau neurofibrillary inclusions were found in only one dog. They were identified with antibodies used to detect tau neurofibrillary lesions in the human brain. The inclusions were also positive for Gallyas silver staining. As in humans, they were distributed mainly in the entorhinal cortex, hippocampus, and temporal cortex. On the other hand, FUS and TDP43 aggregates were not present in any of the examined brain samples. We also found that CDS was characterized by the presence of reactive and senescent microglial cells in the frontal cortex. Our transcriptomic study revealed a significant dysregulation of genes involved in neuroinflammation. Finally, we analyzed tau phosphoproteome in the synaptosomes. Proteomic studies revealed a significant increase of hyperphosphorylated tau in synaptosomes of demented dogs compared with nondemented dogs. This study suggests that cognitive decline in dogs is related to the tau synaptic impairment and neuroinflammation. J. Comp. Neurol. 524:874-895, 2016. © 2015 Wiley Periodicals, Inc.

Oxidative Stress and Protein Quality Control Systems in the Aged Canine Brain as a Model for Human Neurodegenerative Disorders

Aged dogs are considered the most suitable spontaneous animal model for studying normal aging and neurodegenerative diseases. Elderly canines naturally develop cognitive dysfunction and neuropathological hallmarks similar to those seen in humans, especially Alzheimer's disease-like pathology. Pet dogs also share similar living conditions and diets to humans. Oxidative damage accumulates in the canine brain during aging, making dogs a valid model for translational antioxidant treatment/prevention studies. Evidence suggests the presence of detective protein quality control systems, involving ubiquitin-proteasome system (UPS) and Heat Shock Proteins (HSPs), in the aged canine brain. Further studies on the canine model are needed to clarify the role of age-related changes in UPS activity and HSP expression in neurodegeneration in order to design novel treatment strategies, such as HSP-based therapies, aimed at improving chaperone defences against proteotoxic stress affecting brain during aging.

Aging in the canine and feline brain

Aging dogs and cats show neurodegenerative features that are similar to human aging and Alzheimer disease. Neuropathologic changes with age may be linked to signs of cognitive dysfunction both in the laboratory and in a clinic setting. Less is known about cat brain aging and cognition and this represents an area for further study. Neurodegenerative diseases such as lysosomal storage diseases in dogs and cats also show similar features of human aging, suggesting some common underlying pathogenic mechanisms and also suggesting pathways that can be modified to promote healthy brain aging.

Prevention approaches in a preclinical canine model of Alzheimer's disease: benefits and challenges

Aged dogs spontaneously develop many features of human aging and Alzheimer's disease (AD) including cognitive decline and neuropathology. In this review, we discuss age-dependent learning tasks, memory tasks, and functional measures that can be used in aged dogs for sensitive treatment outcome measures. Neuropathology that is linked to cognitive decline is described along with examples of treatment studies that show reduced neuropathology in aging dogs (dietary manipulations, behavioral enrichment, immunotherapy, and statins). Studies in canine show that multi-targeted approaches may be more beneficial than single pathway manipulations (e.g., antioxidants combined with behavioral enrichment). Aging canine studies show good predictive validity for human clinical trials outcomes (e.g., immunotherapy) and several interventions tested in dogs strongly support a prevention approach (e.g., immunotherapy and statins). Further, dogs are ideally suited for prevention studies as they the age because onset of cognitive decline and neuropathology strongly support longitudinal interventions that can be completed within a 3-5 year period. Disadvantages to using the canine model are that they lengthy, use labor-intensive comprehensive cognitive testing, and involve costly housing (almost as high as that of non-human primates). However, overall, using the dog as a preclinical model for testing preventive approaches for AD may complement work in rodents and non-human primates.

A canine model of human aging and Alzheimer's disease

The aged dog naturally develops cognitive decline in many different domains (including learning and memory) but also exhibits human-like individual variability in the aging process. The neurobiological basis for cognitive dysfunction may be related to structural changes that reflect neurodegeneration. Molecular cascades that contribute to degeneration in the aging dog brain include the progressive accumulation of beta-amyloid (Aβ) in diffuse plaques and in the cerebral vasculature. In addition, neuronal dysfunction occurs as a consequence of mitochondrial dysfunction and cumulative oxidative damage. In combination, the aged dog captures key features of human aging, making them particularly useful for the development of preventive or therapeutic interventions to improve aged brain function. These interventions can then be translated into human clinical trials. This article is part of a Special Issue entitled: Animal Models of Disease.

Therapeutic interventions targeting Beta amyloid pathogenesis in an aging dog model

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.

Neurobiology of the aging dog

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.

Aging and cerebrovascular dysfunction: contribution of hypertension, cerebral amyloid angiopathy, and immunotherapy

Age-related cerebrovascular dysfunction contributes to ischemic stroke, intracerebral hemorrhages (ICHs), microbleeds, cerebral amyloid angiopathy (CAA), and cognitive decline. Importantly, there is increasing recognition that this dysfunction plays a critical 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 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. Although 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 β-amyloidosis in the cerebral vasculature.

Acetyl-L-carnitine and alpha-lipoic acid supplementation of aged beagle dogs improves learning in two landmark discrimination tests

Beagle dogs between 7.6 and 8.8 years of age administered a twice daily supplement of alpha-lipoic acid (LA) and acetyl-L-carnitine (ALC) over approximately 2 months made significantly fewer errors in reaching the learning criterion on two landmark discrimination tasks compared to controls administered a methylcellulose placebo. Testing started after a 5 day wash-in. The dogs were also tested on a variable delay version of a previously acquired spatial memory task; results were not significant. The improved performance on the landmark task of dogs supplemented with LA + ALC provides evidence of the effectiveness of this supplement in improving discrimination and allocentric spatial learning. We suggest that long-term maintenance on LA and ALC may be effective in attenuating age-associated cognitive decline by slowing the rate of mitochondrial decay and cellular aging.

Enhanced spatial ability in aged dogs following dietary and behavioural enrichment

We examined the benefits of a broad spectrum antioxidant diet and enrichment comprised of physical exercise, environmental stimulants and cognitive testing, on spatial memory performance in beagle dogs. Both aged (N=48) and young (N=16) beagle dogs (Canus familiaris) were tested yearly on a three-component delayed non-match to position spatial task for three consecutive years. The results showed that young enriched animals acquired the task in fewer sessions, made fewer errors, responded slower and made fewer positional responses, compared to aged enriched animals. An analysis restricted to aged animals revealed that antioxidant administration and enrichment resulted in fewer errors, slower responses and decreased positional responses, particularly in Year 3. Finally, cohort differences emerged, which exemplify the significance of early environmental intervention. Aged dogs that were housed with other animals and exposed to an outdoor environment in early development displayed greater benefits from both interventions. These findings indicate that long-term dietary intervention and enrichment can buffer age-associated cognitive decline.

Microcebus murinus: a useful primate model for human cerebral aging and Alzheimer's disease?

Age-associated dementia, in particular Alzheimer's disease (AD), will be a major concern of the 21st century. Research into normal brain aging and AD will therefore become increasingly important. As for other areas of medicine, the availability of good animal models will be a limiting factor for progress. Given the complexity of the human brain, the identification of appropriate primate models will be essential to further knowledge of the disease. In this review, we describe the features of brain aging and age-associated neurodegeneration in a small lemurian primate, the Microcebus murinus, also referred to as the mouse lemur. The mouse lemur has a relatively short life expectancy, and animals over 5 years of age are considered to be elderly. Among elderly mouse lemurs, the majority show normal brain aging, whereas approximately 20% develop neurodegeneration. This Microcebus age-associated neurodegeneration is characterized by a massive brain atrophy, abundant amyloid plaques, a cytoskeletal Tau pathology and a loss of cholinergic neurons. While elderly mouse lemurs with normal brain aging maintain memory function and social interaction, animals with age-associated neurodegeneration lose their cognitive and social capacities and demonstrate certain similarities with age-associated human AD. We conclude that M. murinus is an interesting primate model for the study of normal brain aging and the biochemical dysfunctions occurring in age-associated neurodegeneration. Mouse lemurs might also become an increasingly important model for the development of novel treatments in this domain.

Effects of lipids and aging on the neurotoxicity and neuronal loss caused by intracerebral injections of the amyloid-β peptide in the rat

The influence of diet and age on the area of lesion and on the neuronal density in the cerebral cortex was studied in rats following local injections of the amyloid-beta peptide (Abeta1-40) in PBS vehicle into the left frontal and cingulate cortices and compared with effects of injections of PBS alone into the corresponding regions of the right hemisphere The experiments were carried out in two groups of animals: one group of young adult rats and a second group of aged rats. Each group of animals, depending on the diet received, was divided into high-cholesterol, high-fat, and a control group. In order to evaluate the interaction of Abeta/PBS-cholesterol and of Abeta/PBS-fat, animals without dietary manipulation receiving Abeta and PBS injection were used as controls. The results showed that the greatest area of lesion was at Abeta injection sites in the high-cholesterol fed group of aged animals. The results also revealed a significant variance in the neuronal density by group and by injection type. Thus, high-cholesterol fed animals showed a greater reduction in neuronal density at Abeta and PBS-injected sites than that seen in the high-fat or control groups. The results also indicate that the loss of neurons at the Abeta injection site exceeds that seen in the PBS-injected area. The greatest reduction in the neuronal density was found at Abeta-injected site in the high-cholesterol fed group of aged animals. In conclusion, our findings indicate an interaction between lipids, age, and Abeta neurotoxicity, and might provide insights into the basic mechanisms involved in a short-term (acute-to-subchronic) response to Abeta peptide.

Learning ability in aged beagle dogs is preserved by behavioral enrichment and dietary fortification: a two-year longitudinal study

The effectiveness of two interventions, dietary fortification with antioxidants and a program of behavioral enrichment, was assessed in a longitudinal study of cognitive aging in beagle dogs. A baseline protocol of cognitive testing was used to select four cognitively equivalent groups: control food-control experience (C-C), control food-enriched experience (C-E), antioxidant fortified food-control experience (A-C), and antioxidant fortified food-enriched experience(A-E). We also included two groups of young behaviorally enriched dogs, one receiving the control food and the other the fortified food. Discrimination learning and reversal was assessed after one year of treatment with a size discrimination task, and again after two years with a black/white discrimination task. The four aged groups were comparable at baseline. At one and two years, the aged combined treatment group showed more accurate learning than the other aged groups. Discrimination learning was significantly improved by behavioral enrichment. Reversal learning was improved by both behavioral enrichment and dietary fortification. By contrast, the fortified food had no effect on the young dogs. These results suggest that behavioral enrichment or dietary fortification with antioxidants over a long-duration can slow age-dependent cognitive decline, and that the two treatments together are more effective than either alone in older dogs.

Amyloid beta peptide-induced cholinergic fibres loss in the cerebral cortex of the rat is modified by diet high in lipids and by age

The influence of diet and age on the effects of intracerebral injection of beta-amyloid peptide (Abeta1-40) in vehicle phosphate-buffered saline (PBS) and on the effects of vehicle alone on cholinergic fibres of the cerebral cortex was studied in rats. The experiments were carried in two groups of animals: one group of young adult rats and a second group of aged rats. Each group of animals, depending on the diet received, was divided into high-cholesterol, high-fat, and a control diet group. In order to evaluate the interaction of Abeta/PBS-cholesterol and of Abeta/PBS-fat, animals without dietary manipulation receiving Abeta and PBS injection were used as controls. High-cholesterol fed animals showed a statistically significant reduction of 49.62% in the number of cholinergic fibres at the Abeta injection site as compared with that at PBS injection site, while the high-fat and control animals showed a significant reduction of 28.13 and 26.81%, respectively. In all diet groups, the loss of cholinergic fibres caused by Abeta as compared to that caused by PBS injection was significantly greater in aged rats in comparison with that observed in the young animals. Furthermore, the results of a multivariate linear regression model revealed that the greatest reduction in cholinergic fibres was in the high-cholesterol fed animals (35 fibres/mm) as compared with that seen in the high-fat and control animals. A significantly greater reduction was also observed at Abeta injection site (28 fibres/mm) as compared with that caused by PBS injection, and a reduction of 16 cholinergic fibres per mm was found in aged animals as compared to that seen in young adult rats. These results show that high-cholesterol diet enhances the toxicity of Abeta peptide and that this is also age-dependent. Therefore, this study increases the evidences of the role of cholesterol in the pathology of Alzheimer's disease (AD).

Structure and function of amyloid in Alzheimer's disease

This review is focused on the structure and function of Alzheimer's amyloid deposits. Amyloid formation is a process in which normal well-folded cellular proteins undergo a self-assembly process that leads to the formation of large and ordered protein structures. Amyloid deposition, oligomerization, and higher order polymerization, and the structure adopted by these assemblies, as well as their functional relationship with cell biology are underscored. Numerous efforts have been directed to elucidate these issues and their relation with senile dementia. Significant advances made in the last decade in amyloid structure, dynamics and cell biology are summarized and discussed. The mechanism of amyloid neurotoxicity is discussed with emphasis on the Wnt signaling pathway. This review is focused on Alzheimer's amyloid fibrils in general and has been divided into two parts dealing with the structure and function of amyloid.

Canine counterpart of senile dementia of the Alzheimer type: amyloid plaques near capillaries but lack of spatial relationship with activated microglia and macrophages

Senile plaques and cerebrovascular amyloidosis are major histopathological lesions in the brains of aged dogs. Different types of amyloid beta protein (A beta) positive plaques are known: diffuse ones and neuritic plaques. Diffuse plaques may contain membrane-bound A beta and/or small amounts of amyloid fibrils. Neuritic plaques are cored plaques with clusters of amyloid fibrils and degenerating neurities. In human amyloid plaques, a pathogenetic role for microglia cells has been described. The aim of this investigation was to study microglia cells in relationship to canine plaques and to investigate the localisation of amyloid plaques in relationship to vasculature. The lesions were studied by hematoxylin and eosin Congo red staining and immunohistochemistry with anti-A beta for plaques, with Mac 387, anti lysozyme and a series of lectins for mononuclear cells, with anti von Willebrand Factor and Lycopersicon esculentum (tomato) lectin for the endothelium of brain capillaries. Diffuse A beta-positive plaques were found in dogs of 10.8 years and older, and cored A beta-positive plaques with birefringent amyloid in Congo red-stained sections in subjects of 15 years and older. Accumulation of microglia cells in relationship to the plaques was not obvious. With anti A beta 8-17 the distribution of the plaques in the cortical layers varied. The younger dogs had primarily diffuse plaques in the deeper layers of the cortical grey matter. The older dogs showed more cored plaques than diffuse plaques which were found throughout all cortical grey matter layers. With anti A beta x-42 more plaques were found positive, especially diffuse ones, whereas staining results of anti A beta x-40 were more confined to amyloid plaques and vascular amyloid. A close spatial relationship was found between the cored plaques and capillaries.

Immunohistochemical investigation of the brain of aged dogs. I. Detection of neurofibrillary tangles and of 4-hydroxynonenal protein, an oxidative damage product, in senile plaques

In the aging dog brain lesions develop spontaneously. They share some morphological characteristics with those of Alzheimer 's disease in man. Diffuse and primitive plaques are well known, whereas neuritic plaques rarely develop. Neurofibrillary tangles have not been seen in the canine. The aim of the present investigation was to study major age-related changes of the dog's brain using paraffin sections with respect to cross-immunoreactivity of tau, A beta protein and other immunoreactive components including hydroxynonenal protein, which is a marker for oxidative damage. The occurrence of neurofibrillary tangles and of the protein tau therein was studied in serial brain sections of two dogs with the Gallyas stain and by immunohistochemistry with three different antibodies against tau. Senile plaques were stained with a monoclonal anti-A beta (residues 8-17), polyclonal anti-apolipoprotein E and a monoclonal antibody against 4-hydroxynonenal (HNE). Amyloid deposits and controls were screened by Congo red staining viewed in fluorescent light, followed by polarized light for green birefringence. With the Gallyas stain and one of the antisera against tau, neurofibrillary tangles were revealed in a similar dispersed pattern, whereas the other antitau antisera gave negative results. With the anti-HNE a positive reaction was found in cerebral amyloid deposits and in vascular wall areas where amyloid deposition was confirmed by Congo-red staining, and in perivascular cells and in some neurons. These results indicate that the canine with his tangles and plaques which show oxidative changes, forms a spontaneous modelfor understanding the early changes and their interrelationships in Alzheimer's disease.

Ultrastructural evidence of fibrillar beta-amyloid associated with neuronal membranes in behaviorally characterized aged dog brains

The aged dog brain accumulates beta-amyloid in the form of diffuse senile plaques, which provides a potentially useful in vivo model system for studying the events surrounding the deposition of beta-amyloid. We used postembedding immunocytochemistry at the electron microscopic level to determine the subcellular distribution of beta-amyloid 1-40 and beta-amyloid 1-42 peptides in the prefrontal and parietal cortex of behaviorally characterized dogs ranging in age from one to 17 years. Immunogold particles signaling beta-amyloid 1-42 occurred over intracellular and extracellular fibrils that were approximately 8 nm in width. Intracellular beta-amyloid 1-42 fibrils were found in close proximity to glial fibrillary acidic protein fibers within astrocytes, but only in cells with signs of plasma membrane disruption. Neuronal labeling of beta-amyloid 1-42 appears to be associated with the plasma membrane. Membrane-bound beta-amyloid 1-42 occurs in the form of fine fibrils that are embedded in the dendritic membrane and appear to project into the extracellular space as determined by quantitative analysis of the immunogold particle distribution. Bundles of beta-amyloid 1-42 were also closely associated and/or integrated with degenerating myelin sheaths of axons. In one dog that was impaired on several cognitive tasks, extensive beta-amyloid 1-42 deposition was associated with microvacuolar changes and vascular pathology. The present findings suggest that beta-amyloid 1-42 may be generated at the dendritic plasma membrane as well as in intracellular compartments. The close association between beta-amyloid 1-42 and destroyed myelin suggests one possible new mechanism by which beta-amyloid 1-42 induces neurodegeneration.

Region-specific age at onset of beta-amyloid in dogs

Cortical patterns of beta-amyloid (Abeta) deposition were evaluated in 40 beagle dogs ranging in age from 2 to 18 years. Abeta deposition in the prefrontal, occipital, parietal and entorhinal cortices was visualized by using an antibody against Abeta1-42. A logistic regression was used to estimate differences in age-at-onset and rate of deposition of Abeta as a function of brain region. The earliest and most consistent site of Abeta deposition with age was in the prefrontal cortex. Entorhinal Abeta deposition was not consistently observed until the age of 14 years, but was present in a subset of dogs under the age of 14 years. These regional vulnerabilities to Abeta accumulation are similar to those seen in the aging human. By using parameters derived from regression analyses, it may be possible to predict the presence of Abeta within specific brain regions in individual dogs. We propose that these models will be a useful tool to evaluate interventions that delay the age of onset or slow the rate of accumulation of Abeta in the dog.

Age-related changes in the brain of the dog

Although many age-related changes have been described in the nervous system of different species, few authors have specifically studied the topic. Knowledge of such changes is essential to veterinary pathologists, who must distinguish the lesions of specific pathologic processes from those arising as a result of normal aging. The brains of 20 old dogs, ranging in age from 8 to 18 years, were compared with those of 10 young dogs using routine staining techniques (hematoxilin and eosin, periodic acid-Schiff), special staining techniques (periodic acid-methenamine silver stain), and immunohistochemical techniques to detect glial fibrillary acid protein, neurofilaments, ubiquitin, and beta-amyloid. Changes affected meninges and choroid plexuses, meningeal and parenchymal vessels, neurons, and glial cells. Of special interest was the presence of polyglucosan bodies, cerebrovascular amyloid deposition, senile plaques, and ubiquitinated bodies. Some of the age-related changes found, particularly lipofuscin, polyglucosan bodies, and beta-amyloid protein deposition, may play a role in the pathogenesis of the canine cognitive dysfunction syndrome. The dog could be used as a natural animal model for the study of normal aging and human neurodegenerative diseases.

Neurofibrillary tangles and Alzheimer's disease

The neuropathological diagnosis of Alzheimer's disease relies on the presence of both neurofibrillary tangles and senile plaques. The number of neurofibrillary tangles is tightly linked to the degree of dementia, suggesting that the formation of neurofibrillary tangles more directly correlates with neuronal dysfunction. The regional pattern of areas affected by neurofibrillary tangles formation during the course of the disease is relatively stereotyped. Neurofibrillary tangles are composed of highly phosphorylated forms of the microtubule-associated protein tau. Phosphorylated tau proteins accumulate early in neurons, even before formation of neurofibrillary tangles, suggesting that an imbalance between the activities of protein kinases and phosphatases acting on tau is an early phenomenon. The latter might be related to changes in signalling through transduction cascades, since many of the protein kinases generating phosphorylated tau species participate in signalling pathways. The accumulation of neurofibrillary tangles and phosphorylated tau species is associated with disturbances of the microtubule network and, as a consequence of the latter, of axoplasmic flows. The mechanistic relationship between the formation of neurofibrillary tangles and senile plaques is still little understood and in vivo formation of neurofibrillary tangles in experimental models has not yet been achieved. Future animal models, e.g. transgenic animals expressing combined key human proteins, will hopefully reproduce faithfully all the major cellular lesions of the disease.

Animal models of cerebral beta-amyloid angiopathy

Cerebral amyloid angiopathy (CAA) is a significant risk factor for hemorrhagic stroke in the elderly, and occurs as a sporadic disorder, as a frequent component of Alzheimer's disease, and in several rare, hereditary conditions. The most common type of amyloid found in the vasculature of the brain is beta-amyloid (A beta), the same peptide that occurs in senile plaques. A paucity of animal models has hindered the experimental analysis of CAA. Several transgenic mouse models of cerebral beta-amyloidosis have now been reported, but only one appears to develop significant cerebrovascular amyloid. However, well-characterized models of naturally occurring CAA, particularly aged dogs and non-human primates, have contributed unique insights into the biology of vascular amyloid in recent years. Some non-human primate species have a predilection for developing CAA; the squirrel monkey (Saimiri sciureus), for example, is particularly likely to manifest beta-amyloid deposition in the cerebral blood vessels with age, whereas the rhesus monkey (Macaca mulatta) develops more abundant parenchymal amyloid. These animals have been used to test in vivo beta-amyloid labeling strategies with monoclonal antibodies and radiolabeled A beta. Species-differences in the predominant site of A beta deposition also can be exploited to evaluate factors that direct amyloid selectively to a particular tissue compartment of the brain. For example, the cysteine protease inhibitor, cystatin C, in squirrel monkeys has an amino acid substitution that is similar to the mutant substitution found in some humans with a hereditary form of cystatin C amyloid angiopathy, possibly explaining the predisposition of squirrel monkeys to CAA. The existing animal models have shown considerable utility in deciphering the pathobiology of CAA, and in testing strategies that could be used to diagnose and treat this disorder in humans.

Amyloid deposits in the gastrointestinal tract of aging dogs

Pathologic features, immunohistochemical characteristics, and incidence of gastrointestinal amyloid deposition were investigated in 78 canine necropsies, including 43 males and 35 females ranging from 2 months to 20 years of age. In 31 of 78 dogs, amyloid protein was accumulated in or around vessel walls located in submucosal and muscular layers of the alimentary tract and mesenteric tissues. These deposits did not seem to affect the function(s) of the digestive tract histologically. The deposits showed congophilia and green birefringence under polarized microscopy. Transmission electron microscopy revealed randomly arranged amyloid fibrils 6-10 nm in diameter around a blood vessel. Congophilia was retained even after oxidation with potassium permanganate, suggesting that this protein is not the AA form of amyloid. Although immunohistochemical studies were performed using antibodies to canine amyloid apolipoprotein AI and synthetic human beta amyloid peptide, the immunochemical nature of the protein was not determined. The mean ages of both male and female dogs with gastrointestinal amyloid deposition were higher than those of their normal counterparts (P < 0.001), and the incidence of deposition in elderly dogs was higher than that in young dogs (P < 0.005). However, there was no sex predilection with respect to both the mean age of the dogs with deposition and the incidence of occurrence. The mechanism(s) and importance of this form of amyloid deposition remain unclear.

Subpopulation of dogs with severe brain parenchymal beta amyloidosis distinguished with cluster analysis

A study of the brains of 30 dogs, mongrels from 6.5 to 26.5 years of age, revealed that all dogs older than 13 years of age develop amyloid-beta-positive plaques. Cluster analysis based on the age of the dogs and the numerical density of amyloid-positive plaques stained with monoclonal antibody 4G8 (17-24aa) revealed that the population of old dogs consists of two subpopulations: one with a very low (0.8/mm2 on average) and other with a high (19.2/mm2 on average) numerical density of plaques. These two groups (19.5 and 19.1 years of age, respectively) appear to emerge from the younger group (12.2 years of age on average), with moderate (2.2/mm2 on average) numerical density of 4G8-positive plaques. These data may indicate that only a portion of the mongrel population (43%) is susceptible to amyloidosis beta or that only this severely affected subpopulation was exposed to a factor or factors inducing this pathology and developed severe cortical amyloidosis that correlates with age. Dog plaques are only of the diffuse type, with nonfibrillar, thioflavin S-, and Congo red-negative amyloid in all groups distinguished by cluster analysis. Only from 10% of 4G8-positive plaques in the mildly affected group to 29% in the severely and 37% in the moderately affected group are Bielschowsky positive. In the younger, moderately affected group, 6E10 (1-17aa)-positive plaques prevail. In the two old groups with severe and weak changes, almost all 4G8-positive plaques are also 6E10-positive. Carboxy-terminal region immunocytochemistry reveals that BC42-positive plaques are numerous, whereas BC40-positive plaques are few or absent. The differences in the silver-positivity of plaques and their immunoreactivity in both the amino- and carboxy-terminal regions may reflect differences in amyloid-beta deposition and resolution. Dog parenchymal amyloidosis beta appears to be a model for the study of diffuse plaques.

The canine as an animal model of human aging and dementia

The aged canine displays many features that make it an excellent model for studying the progression of pathology in brain aging and linking these findings to learning, memory and other cognitive functions. Canines develop extensive beta-amyloid deposition within neurons and their synaptic fields, which appears to give rise to senile plaques. These plaques are primarily of the early diffuse subtype. Aged canines also exhibit accumulations of lipofuscin, cerebral vascular changes, dilation of the ventricles, and cytoskeletal changes. Neurofibrillary tangles (NFTs) are not present in the aged canine. Thus, the aged canine brain provides a suitable model for studying early degeneration normally considered to be pre-Alzheimer's. This supposition is also supported by behavioral data. We have found that the extent of beta-amyloid deposition correlates with a decline in select measures of cognitive function. These data provide the first evidence of a correlation between beta-amyloid accumulation and cognitive decline in the absence of NFTs. We summarize four lines of evidence that support using the aged canine as a model of human aging: (a) Aged canines develop aspects of neuropathology similar to that observed in aged humans; (b) Veterinarians have observed that many canines exhibit a clinical syndrome of age-related cognitive dysfunction; (c) Aged canines are deficient on a variety of neuropsychological tests of cognitive function; (d) The level of beta-amyloid accumulation correlates with cognitive dysfunction in the canine. These data indicate that the aged canine is a particularly useful model for studying age-related cognitive dysfunction (ARCD), early neuronal changes associated with aging, and the initial stages of senile plaque formation.

Age-specific onset of beta-amyloid in beagle brains

As part of an effort to characterize Alzheimer's disease-like neuropathy in the canine brain we have determined the age of onset of spontaneous beta-amyloid deposition in 103 laboratory-raised beagles. Tissue samples for each subject were obtained from hippocampal and cortical regions and examined for the incidence and density of beta-amyloid deposition after staining with modified Bielschowsky silver stain and immunohistochemistry. Amyloid deposition was characterized as diffuse plaque or cloud-like formation. The diffuse type of beta-amyloid plaque formation predominated in all brain regions examined. A threshold effect of plaque development was observed; no plaques were apparent in dogs before the age of 10 years, while 36% of dogs aged 11.1-12.9, 60% of dogs aged 13.0-15.0, and 73% of dogs aged 15.1-17.8 developed beta-amyloid deposits. Additionally, a significant increase in plaque density was observed with increasing age.

Carboxy terminal of beta-amyloid deposits in aged human, canine, and polar bear brains

Immunocytochemistry, using antibodies specific for different carboxy termini of beta-amyloid. A beta 40 and A beta 42(43), was used to compare beta-amyloid deposits in aged animal models to nondemented and demented Alzheimer's disease human cases. Aged beagle dogs exhibit diffuse plaques in the absence of neurofibrillary pathology and the aged polar bear brains contain diffuse plaques and PHF-1-positive neurofibrillary tangles. The brains of nondemented human subjects displayed abundant diffuse plaques, whereas the AD cases had both diffuse and mature (cored) neuritic plaques. Diffuse plaques were positively immunostained with an antibody against A beta 42(43) in all examined species, whereas A beta 40 immunopositive mature plaques were observed only in the human brain. Anti-A beta 40 strongly immunolabeled cerebrovascular beta-amyloid deposits in each of the species examined, although some deposits in the polar bear brain were preferentially labeled with anti-A beta 42(43). beta-amyloid deposition was evident in the outer molecular layer of the dentate gyrus in the aged dog, polar bear, and human. Within this layer, A beta 42 was present as diffuse deposits, although these deposits were morphologically distinct in each of the examined animal models. In dogs, A beta 42 was cloud-like in nature; the polar bear demonstrated a more aggregated type of deposition, and the nondemented human displayed well-defined deposits. Alzheimer's disease cases were most frequently marked by neuritic plaques in this region. Taken together, the data indicate that beta-amyloid deposition in aged mammals is similar to the earliest stages observed in human brain. In each species, A beta 42(43) is the initially deposited isoform in diffuse plaques.

The older rabbit as an animal model: implications for Alzheimer's disease

Eyeblink classical conditioning (EBCC) is impared in rabbits and humans during normal aging and severely disrupted in Alzheimer's disease (AD) and older Down's Syndrome patients (called DS/AD). To determine if older rabbit brains developed neuropathological evidence of Alzheimer-like pathology to account for impaired EBCC, the cerebellum and hippocampus of behaviorally tested rabbits aged 3 months to 7 years were probed using immunohistochemical techniques. Significant cell loss and gliosis were observed in some brain regions, but there was little or no deposition of beta-amyloid (A beta) or abnormal tau accumulations in telencephalic neurons, even in rabbits over 7 years of age. Our aims here are to: 1) report the results of our search for Alzheimer-like neuropathology in aged rabbit brains; and 2) highlight similarities in the brain mechanisms for EBCC between rabbits and humans and, hence, the utility of studies of EBCC in rabbits as a model system for testing cognition-enhancing drugs.

Axonal degeneration promotes abnormal accumulation of amyloid beta-protein in ascending gracile tract of gracile axonal dystrophy (GAD) mouse

The GAD mouse is a spontaneous neurological mutant with axonal dystrophy in the gracile tract of the medulla oblongata and spinal cord. The immunoreactivity of amyloid precursor protein (APP-IR) and amyloid beta-protein (A beta P-IR) was examined in the gracile tract and the dorsal root ganglia of normal and GAD mice. The mice were studied at 4, 9, 18, and 32 weeks of age. These periods correspond clinically to the initial, progressive, critical, and terminal stages of the disease, respectively. The APP-IR in both axons and glial cells was already accentuated to a higher level as early as 4 weeks of age in the gracile nucleus of GAD mouse. Similarly there was increase in APP-IR of GAD mouse in the dorsal root ganglia. Almost all of the primary neurons in the dorsal root ganglia at the lumbar cord level of GAD mouse revealed stronger APP-IR than those of normal mouse throughout all stages. The cells showing immunoreactivity for amyloid beta-protein became positive in axons and glial cells in the gracile nucleus by approximately the 9th week, and followed by an increase of A beta P-IR in order of the cervical, thoracic and lumbar spinal cords. These results suggest that the initial feature in GAD mouse is an accumulation of amyloid precursor protein induced by axonal dystrophy which then leads to a deposition of amyloid beta-protein within the cytoplasm of both axons and glial cells in the gracile tract.

Age-related changes of mRNA expression of amyloid precursor protein in the brain of senescence-accelerated mouse

APP695 mRNA is only expressed in the brains of SAM. The expression of APP mRNA in SAM P1 mice brains is more marked than that in SAM R1 mice brain. APP mRNA expression was increased with advancing age in all brain regions of SAM P1 mice compared with SAM R1. Especially, the changes of the amount of APP mRNA in the prosencephalon and the mesencephalon are significant at P value of 0.05. We suggest that overexpression of APP mRNA may be related to accelerated aging phenomenon in the SAM brain. This is the first report of age-related increase in the amount of APP mRNA in the SAM brain.

Transgenic mice and modeling Alzheimer's disease

The etiology of Alzheimer's disease (AD) is poorly understood, and no effective therapies are available. Although histopathology of the disease has been studied thoroughly, the relationship of various AD lesions to pathological processes and to dementia are debated. Progress would be greatly enhanced by existence of manipulable small animal models of the disease. Recently, transgenic strategies to developing such a model have been extensively explored. The approach has proved to be difficult and has yielded some disappointments, but also some encouraging results. Transgenic strategies for obtaining a model for AD are surveyed in this review and, as an illustration, early AD-like features of transgenic mice produced in our laboratory are described.

The spatial patterns of beta/A4 deposit subtypes in Down's syndrome

The spatial patterns of diffuse, primitive and classic beta/A4 deposits were studied in coronal sections of the hippocampus and adjacent gyri in 11 cases of Down's syndrome (DS) varying in age from 38 to 67 years. The objectives of the study were first, to compare the spatial patterns of beta/A4 deposits revealed in DS with those reported in cases of Alzheimer's disease (AD) and second, to study how the spatial patterns of beta/A4 deposits may develop in the tissue. The spatial patterns revealed in DS exhibited a number of similarities with those reported in AD: (1) the range and frequency of the different types of spatial pattern revealed were similar, (2) beta/A4 deposits occurred in clusters and in many cortical tissues, the clusters were distributed in a regular pattern parallel to the pia, (3) the clusters of diffuse and primitive beta/A4 deposits occurred in an alternating pattern along the cortex, and (4) the clusters of classic beta/A4 deposits were not correlated with the clusters of the diffuse and primitive deposits. Primitive deposits may develop from the diffuse deposits in regions of the cortex where extracellular paired helical filaments were formed. However, clusters of the classic beta/A4 deposits, which are formed in older cases, appear to develop independently of the diffuse and primitive deposits.

Transgenic mouse brain histopathology resembles early Alzheimer's disease

Transgenic mice expressing the 751-amino acid form of the human amyloid precursor protein develop extracellular beta-amyloid protein (A beta)-immunoreactive deposits that increase in frequency with age. Here we show that the appearance and histological profile of deposits in the transgenic mice closely resemble those of preamyloid deposits in the brains of young adults with Down's syndrome, who presumably have the pathology of early-stage Alzheimer's disease. Specific monoclonal antibodies reveal that material in the deposits has the free carboxyl terminus of A beta 1-42, and that the deposits contain material which, by immunohistochemical analysis, apparently originates from the human beta-amyloid precursor protein (beta PP) transgene. In rare cases, the transgenic mouse brains contain several different histopathological characteristics of Alzheimer lesions. These features include dense A beta immunoreactivity which co-localizes with gliosis and with Alz50-immunoreactive structures resembling swollen boutons of dystrophic neurites. These observations demonstrate that the murine brain is capable of reproducing several typical features of Alzheimer histopathology.

Abnormally phosphorylated tau protein related to the formation of neurofibrillary tangles and neuropil threads in the cerebral cortex of sheep and goat

Frontal sections including temporal isocortex, entorhinal region and hippocampus from aged domestic animals (dog, cat, horse, sheep and goat) were studied for Alzheimer-related changes using immunostaining with the AT8 antibody for abnormally phosphorylated tau protein and selective silver techniques for A4 amyloid and neurofibrillary changes of the Alzheimer type. The material available to us did not show A4 amyloid deposits or argyrophilic neurofibrillary changes. Only the brains of aged sheep and goat exhibited the presence of AT8-immunoreactive pyramidal cells in the entorhinal region and hippocampal formation. Two groups of AT8-positive neurons could be observed: The first group contained evenly distributed immunoreactive material in all parts of the soma, the dendrites and the axon. The neuronal processes appeared quite normal. The second group, however, showed conspicuous changes in the cellular processes consisting of a loss of immunoreactivity within the axon and the proximal dendrites and the appearance of intensely stained swellings within the curved distal dendrites. These changes were closely reminiscent to alterations of the cytoskeleton known to occur at the same location in the aging human brain and in Alzheimer's disease. The findings justify a closer look at sheep and goat when searching for suitable animal models for experimental studies of the conditions responsible for the development of Alzheimer-related neurofibrillary changes.

Neurofibrillary tangles in the cerebral cortex of sheep

Neurofibrillary degeneration, including neurofibrillary tangles (NFTs) and neuritic plaques, is an important pathological hallmark of Alzheimer's disease (AD). Unfortunately, no practical animal model of neurofibrillary degeneration has been described. We report here the presence of structures in the cerebral cortex of sheep, Ovis aries, that resemble Alzheimer NFTs and neuritic plaques. NFT-like structures and clusters of degenerating neurites are stained by silver impregnation and thioflavin-S, and are immunoreactive with antibodies against tau microtubule-associated proteins. Viewed under the electron microscope, tau-immunoreactive tangles consist of paired helical filaments. Naturally occurring neurofibrillary structures in sheep cortex provide a model for studying the pathobiology of Alzheimer's disease.

Differences in beta-amyloid (beta/A4) deposition in human patients with Down's syndrome and sporadic Alzheimer's disease

The density of diffuse, primitive, classic and compact beta-amyloid (beta/A4) deposits was estimated in the hippocampus and adjacent gyri in human patients with Down's syndrome (DS) and sporadic Alzheimer's disease (AD). The objective of the study was to determine whether there were differences in beta/A4 deposition in DS and sporadic AD and whether these differences could be attributed to overexpression of the amyloid precursor gene (APP) in DS. Total beta/A4 deposit density was greater in DS than AD in all brain regions studied but the DS/AD density ratios varied between brain regions. In the majority of brain regions, the ratio of primitive to diffuse beta/A4 deposits was greater in DS but the ratio of classic to diffuse deposits was greater in AD. The data were consistent with the hypothesis that overexpression of the APP gene in DS may lead to increased beta/A4 deposition. However, local brain factors also appear to be important in beta/A4 deposition in DS. Overexpression of the APP gene may also be responsible for increased production of paired helical filaments (PHF) and result in enhanced formation of primitive beta/A4 deposits in DS. In addition, increased formation of classic deposits in AD suggests that factors necessary for the production of a compact amyloid core are enhanced in AD compared with DS.

The distribution of amyloid beta precursor protein in canine brain

The distribution of amyloid beta precursor protein (APP) in canine brain was investigated. By immunoblot analysis, APP-positive bands corresponding to proteins of 105-120 kilodalton were recognized in all canine brains regardless of the individual age of the dogs. Bands of similar molecular mass were also detected in the meninges, cerebrospinal fluid, and several visceral organs. Immunohistochemical studies were performed using cryostat and paraffin-embedded sections pretreated with formic acid or by the hydrated autoclave method. In the normal canine brain, APP was found to be distributed in the neurons and vascular system. In the brains with SP, obvious accumulation of APP was observed in swollen neurites within amyloid plaques, although the relationship between APP and diffuse plaques was unclear. APP accumulation in swollen axons was also seen around necrotic foci in the brain of one dog with necrotizing purulent encephalitis. These studies revealed that distribution of APP in canine tissues, especially in the brain, and the accumulation of APP in swollen neurites or axons.

Beta-amyloid accumulation in aged canine brain: a model of early plaque formation in Alzheimer's disease

We characterized eight aged beagles (maintained from birth in a laboratory colony) and one black Labrador using Bielschowsky's, thioflavine S, and Congo red staining, and antibodies to the beta-amyloid peptide, dystrophic neurites, and other plaque components. All plaques within these canine brains were of the diffuse subtype and were neither thioflavine S- nor Congo red-positive. The majority of plaques in the entorhinal cortex contained numerous neurons within them while plaques in the dentate gyrus did not. beta-Amyloid immunoreactivity was also present within select neurons and neuronal processes and was detected as a diffuse linear zone corresponding to the terminal fields of the perforant path. There was no significant correlation between extent of beta-amyloid accumulation and neuron number in entorhinal cortex. Neither tau-1, PHF-1, nor SMI-31-immunostaining revealed dystrophic fibers, confirming the classification of these plaques as diffuse. Canine plaques did not appear to contain bFGF- or HS-positive immunostaining. This may explain why neuritic involvement was not detected within these canine plaques. It is possible that the beta-amyloid within the canine brain has a unique primary structure or may not be in an assembly state that adversely affects neurons.

The spatial patterns of beta/A4 deposit subtypes in Alzheimer's disease

The spatial patterns of diffuse, primitive, classic (cored) and compact (burnt-out) subtypes of beta/A4 deposits were studied in coronal sections of the frontal lobe and hippocampus, including the adjacent gyri, in nine cases of Alzheimer's disease (AD). If the more mature deposits were derived from the diffuse deposits then there should be a close association between their spatial patterns in a brain region. In the majority of tissues examined, all deposit subtypes occurred in clusters which varied in dimension from 200 to 6400 microns. In many tissues, the clusters appeared to be regularly spaced parallel to the pia or alveus. The mean dimension of the primitive deposit clusters was greater than those of the diffuse, classic and compact types. In about 60% of cortical tissues examined, the clusters of primitive and diffuse deposits were not in phase, i.e. they alternated along the cortical strip. Clusters of classic deposits appeared to be distributed independently of the diffuse deposit clusters. Cluster size of the primitive deposits was positively correlated with the density of the primitive deposits in a tissue but no such relationship could be detected for the diffuse deposits. This study suggested that there was a complex relationship between the clusters of the different subtypes of beta/A4 deposits. If the diffuse deposits do give rise to the primitive and classic varieties then factors unrelated to the initial deposition of beta/A4 in the form of diffuse plaques were important in the formation of the mature deposits.

Immunohistochemical and immunoelectron microscopical characterization of cerebrovascular and senile plaque amyloid in aged dogs' brains

Immunohistochemical and immunoelectron microscopical studies were carried out on 28 aged dogs' brains. Amyloid deposits were seen in the arteries and capillaries in the leptomeninges and in superficial areas of the cortices in 19 (67.9%) of the 28 dogs (10-22 years of age). Immunohistochemically, these amyloid deposits were reactive for anti-beta/A4 antibody. Additionally, a variable number of parenchymal deposits with diffuse beta/A4-immunoreactivity (diffuse plaques) was also noted throughout the cerebral cortex in 24/28 dogs (85.7%). However, these plaque lesions were undetectable in Congo red staining. Electron microscopically, amyloid fibrils, measuring 10 nm in width, were located mainly in the tunica media of the arteries, and in less involved vessels they tended to be present among collagen fibres in the adventitia and smooth muscle cells in the outer layer of the media. The plaque lesions appeared to contain sparse aggregations of amyloid fibrils. In immunoelectron microscopical examinations, all amyloid fibrils in both blood vessels and plaques were selectively labelled by gold particles. These findings indicate that aged dogs can provide a useful experimental model for research into the beta/A4-type of cerebral amyloidosis commonly seen in Alzheimer's disease.