Senile plaques in cortex of aged normal monkeys

Senile plaques and phosphorylated tau deposition in a super-aged rhesus monkey (Macaca mulatta)

The brain of a rhesus monkey that died at 43 years of age with symptoms of suspected cognitive dysfunction was analyzed. pathological analyses revealed characteristic Alzheimer's disease-related lesions: the aggregation of amyloid β (Aβ) in the form of senile plaques and phosphorylated tau proteins. We also revealed that Aβ43, which is prone to aggregation and toxicity in humans, is involved in senile plaques in the brain of the rhesus monkey, as well as several other Aβ species. Comparative studies of neuropathology using aged nonhuman primates lack behavioral descriptions compared to human medicine. This case report showed behavioral abnormalities and the detailed pathological changes that may have caused it in a super-aged rhesus monkey.

Genetic Insights into Alzheimer's Disease

Alzheimer's disease (AD) is a pervasive, relentlessly progressive neurodegenerative disorder that includes both hereditary and sporadic forms linked by common underlying neuropathologic changes and neuropsychological manifestations. While a clinical diagnosis is often made on the basis of initial memory dysfunction that progresses to involve multiple cognitive domains, definitive diagnosis requires autopsy examination of the brain to identify amyloid plaques and neurofibrillary degeneration. Over the past 100 years, there has been remarkable progress in our understanding of the underlying pathophysiologic processes, pathologic changes, and clinical phenotypes of AD, largely because genetic pathways that include but expand beyond amyloid processing have been uncovered. This review discusses the current state of understanding of the genetics of AD with a focus on how these advances are both shaping our understanding of the disease and informing novel avenues and approaches for development of potential therapeutic targets.

Photobiomodulation improves the frontal cognitive function of older adults

OBJECTIVES

The frontal lobe hypothesis of age-related cognitive decline suggests that the deterioration of the prefrontal cortical regions that occurs with aging leads to executive function deficits. Photobiomodulation (PBM) is a newly developed, noninvasive technique for enhancing brain function, which has shown promising effects on cognitive function in both animals and humans. This randomized, sham-controlled study sought to examine the effects of PBM on the frontal brain function of older adults.

METHODS/DESIGNS

Thirty older adults without a neuropsychiatric history performed cognitive tests of frontal function (ie, the Eriksen flanker and category fluency tests) before and after a single 7.5-minute session of real or sham PBM. The PBM device consisted of three separate light-emitting diode cluster heads (633 and 870 nm), which were applied to both sides of the forehead and posterior midline, and delivered a total energy of 1349 J.

RESULTS

Significant group (experimental, control) × time (pre-PBM, post-PBM) interactions were found for the flanker and category fluency test scores. Specifically, only the older adults who received real PBM exhibited significant improvements in their action selection, inhibition ability, and mental flexibility after vs before PBM.

CONCLUSIONS

Our findings support that PBM may enhance the frontal brain functions of older adults in a safe and cost-effective manner.

Alzheimer's disease: experimental models and reality

Experimental models of Alzheimer's disease (AD) are critical to gaining a better understanding of pathogenesis and to assess the potential of novel therapeutic approaches. The most commonly used experimental animal models are transgenic mice that overexpress human genes associated with familial AD (FAD) that result in the formation of amyloid plaques. However, AD is defined by the presence and interplay of both amyloid plaques and neurofibrillary tangle pathology. The track record of success in AD clinical trials thus far has been very poor. In part, this high failure rate has been related to the premature translation of highly successful results in animal models that mirror only limited aspects of AD pathology to humans. A greater understanding of the strengths and weakness of each of the various models and the use of more than one model to evaluate potential therapies would help enhance the success of therapy translation from preclinical studies to patients. In this review, we summarize the pathological features and limitations of the major experimental models of AD, including transgenic mice, transgenic rats, various physiological models of sporadic AD and in vitro human cell culture models.

Senile plaques and cerebral amyloid angiopathy in an aged California sea lion (Zalophus californianus)

Senile plaques (SPs) and cerebral amyloid angiopathy (CAA) consisting of β-amyloid (Aβ) are major features in the brain of Alzheimer's disease (AD) patients and elderly humans and animals. In this study, we report the finding of SPs and CAA in an aged sea lion (30 years), which is the first demonstration of AD-related pathological changes in a marine animal. Histologically, SPs were observed at the cerebral cortex, most frequently at the frontal lobe, with two morphologically different types: the small round type and the large granular type. Only the small round SPs were positive for Congo red staining. The SPs were equally immunoreactive to Aβ40 and Aβ42 and were mainly composed of Aβ with an N-terminal pyroglutamate residue at position 3. Amyloid depositions at vessel walls were noted at the meninges and within the parenchyma. Interestingly, double immunofluorescence staining for Aβ40 and Aβ42 showed that the two subtypes were deposited segmentally in different parts of the vessel walls. The lesions observed in the sea lion suggest that Aβ deposition is widely present in various animal species, including marine mammals; however, the peculiar deposits similar to cotton wool plaques and the specific pattern of CAA are characteristic features of this animal.

Transgenic models of Alzheimer's disease: better utilization of existing models through viral transgenesis

Animal models have been used for decades in the Alzheimer's disease (AD) research field and have been crucial for the advancement of our understanding of the disease. Most models are based on familial AD mutations of genes involved in the amyloidogenic process, such as the amyloid precursor protein (APP) and presenilin 1 (PS1). Some models also incorporate mutations in tau (MAPT) known to cause frontotemporal dementia, a neurodegenerative disease that shares some elements of neuropathology with AD. While these models are complex, they fail to display pathology that perfectly recapitulates that of the human disease. Unfortunately, this level of pre-existing complexity creates a barrier to the further modification and improvement of these models. However, as the efficacy and safety of viral vectors improves, their use as an alternative to germline genetic modification is becoming a widely used research tool. In this review we discuss how this approach can be used to better utilize common mouse models in AD research. This article is part of a Special Issue entitled: Animal Models of Disease.

A review of the structural alterations in the cerebral hemispheres of the aging rhesus monkey

Like humans, rhesus monkeys show cognitive decline and this review considers what structural age-related changes underlie this decline. Some structural measures do not alter significantly with age. These include brain weight, overall cortical thickness; numbers of cortical neurons; and numbers of astrocytes and microglial cells. Other structural measures change with age, but the change does not correlate with cognitive decline. These changes include nerve fiber loss from some fiber tracts, degeneration, and regeneration of myelin sheaths, and increase in the frequency of oligodendrocytes. Among the structural measures that increase in frequency with age and also correlate with cognitive decline are the increased frequency of degenerating myelin sheaths and a loss of nerve fibers from some fiber tracts; and the loss of synapses and dendritic spines from upper layers of prefrontal cortex. Consequently, the existing data suggest that cognitive decline correlates with changes in myelinated nerve fibers and with disconnections between and within cortical areas, as reflected by the age-related loss of synapses and of dendritic spines from some cortical areas.

Should Alzheimer's disease be equated with human brain ageing? A maladaptive interaction between brain evolution and senescence

In this review Alzheimer's disease is seen as a maladaptive interaction between human brain evolution and senescence. It is predicted to occur in everyone although does not necessarily lead to dementia. The pathological process is initiated in relation to a senescence mediated functional down-regulation in the posteromedial cortex (Initiation Phase). This leads to a loss of glutamatergic excitatory input to layer II entorhinal cortex neurons. A human specific maladaptive neuroplastic response is initiated in these neurons leading to neuronal dysfunction, NFT formation and death. This leads to further loss of glutamatergic excitatory input and propagation of the maladaptive response along excitatory pathways linking evolutionary progressed vulnerable neurons (Propagation Phase). Eventually neurons are affected in many brain areas resulting in dementia. Possible therapeutic approaches include enhancing glutamatergic transmission. The theory may have implications with regards to how Alzheimer's disease is classified.

Characterization of AβpN3 deposition in the brains of dogs of various ages and other animal species

Senile plaques (SP) are characteristic histopathological manifestations of Alzheimer's disease (AD), but are also found in normal aging (NA). Recent studies have demonstrated that beta amyloid (Aβ) proteins that have been truncated at the N-terminal position 3 (AβpN3) are the predominant component of SP in AD, but not in NA. The present study revealed that AβpN3 was deposited in an age-dependent manner in canine brains. Moreover, AβpN3 was the main component of the SP that developed in very old dogs. The deposition of AβpN3 increased in accordance with the number of SP, but that of N-terminally intact Aβ (AβN1) did not. In addition, AβpN3 was also deposited in the SP of a Japanese macaque and an American black bear, but not in a feline brain. Focal microvascular cerebral amyloid angiopathy was also observed in the deep cortices and the white matter of the dogs and a woodpecker. Those were always composed of both AβpN3 and AβN1. In conclusion, though non-human animals do not develop full pathology of AD of the human type, AβpN3 is widely deposited in the brains of senescent vertebrates.

Acute gamma-secretase inhibition of nonhuman primate CNS shifts amyloid precursor protein (APP) metabolism from amyloid-beta production to alternative APP fragments without amyloid-beta rebound

The accumulation of amyloid beta (Abeta) in Alzheimer's disease is caused by an imbalance of production and clearance, which leads to increased soluble Abeta species and extracellular plaque formation in the brain. Multiple Abeta-lowering therapies are currently in development: an important goal is to characterize the molecular mechanisms of action and effects on physiological processing of Abeta, as well as other amyloid precursor protein (APP) metabolites, in models which approximate human Abeta physiology. To this end, we report the translation of the human in vivo stable-isotope-labeling kinetics (SILK) method to a rhesus monkey cisterna magna ported (CMP) nonhuman primate model, and use the model to test the mechanisms of action of a gamma-secretase inhibitor (GSI). A major concern of inhibiting the enzymes which produce Abeta (beta- and gamma-secretase) is that precursors of Abeta may accumulate and cause a rapid increase in Abeta production when enzyme inhibition discontinues. In this study, the GSI MK-0752 was administered to conscious CMP rhesus monkeys in conjunction with in vivo stable-isotope-labeling, and dose-dependently reduced newly generated CNS Abeta. In contrast to systemic Abeta metabolism, CNS Abeta production was not increased after the GSI was cleared. These results indicate that most of the CNS APP was metabolized to products other than Abeta, including C-terminal truncated forms of Abeta: 1-14, 1-15 and 1-16; this demonstrates an alternative degradation pathway for CNS amyloid precursor protein during gamma-secretase inhibition.

Distribution of neprilysin and deposit patterns of Abeta subtypes in the brains of aged squirrel monkeys (Saimiri sciureus)

Beta-amyloid (Abeta) is deposited in the parenchyma and blood vessel walls of the senescent brain, and forms lesions termed senile plaques (SPs) and cerebral amyloid angiopathy (CAA). Since in Alzheimer's disease (AD) excessive Abeta is linked to cognitive deterioration, the mechanisms of degradation and clearance of Abeta are now being researched for use in AD therapy. We conducted an immunohistochemical study of the patterns of deposition of two Abeta subtypes (Abeta40 and Abeta42) and the distribution of the Abeta degrading enzyme neprilysin (NEP) in the brains of aged squirrel monkeys, a species known to develop CAA and SPs. Abeta deposits were observed mainly in the cerebral cortex of five older monkeys, and were absent in monkeys under 12 years of age. NEP expression was observed in the caudate nucleus, putamen, globus pallidus, substantia nigra and the molecular layer of the dentate gyrus, and thus exhibited a distribution complementary to those of CAA and SPs in cerebral cortex and hippocampus. It is known that CAA is more prominent than SPs in squirrel monkey brains. However, we confirmed that Abeta40 is deposited predominantly in the arterioles of the meninges and penetrates vertically into the cerebral cortex, whereas Abeta42 is deposited predominantly in the capillaries of the cerebral cortex. These distinct patterns of deposition of Abeta subtypes are likely related to the difference in biochemical character of these two subtypes. We have demonstrated for the first time the distribution of NEP in the brain of a non-human primate, the squirrel monkey, which appears useful for research on AD treatment.

Effects of normal aging on prefrontal area 46 in the rhesus monkey

This review is concerned with the effects of normal aging on the structure and function of prefrontal area 46 in the rhesus monkey (Macaca mulatta). Area 46 has complex connections with somatosensory, visual, visuomotor, motor, and limbic systems and a key role in cognition, which frequently declines with age. An important question is what alterations might account for this decline. We are nowhere near having a complete answer, but as will be shown in this review, it is now evident that there is no single underlying cause. There is no significant loss of cortical neurons and although there are a few senile plaques in rhesus monkey cortex, their frequency does not correlate with cognitive decline. However, as discussed in this review, the following do correlate with cognitive decline. Loss of white matter has been proposed to result in some disconnections between parts of the central nervous system and changes in the structure of myelin sheaths reduce conduction velocity and the timing in neuronal circuits. In addition, there are reductions in the inputs to cortical neurons, as shown by regression of dendritic trees, loss of dendritic spines and synapses, and alterations in transmitters and receptors. These factors contribute to alterations in the intrinsic and network physiological properties of cortical neurons. As more details emerge, it is to be hoped that effective interventions to retard cognitive decline can be proposed.

The effects of normal aging on myelinated nerve fibers in monkey central nervous system

The effects of aging on myelinated nerve fibers of the central nervous system are complex. Many myelinated nerve fibers in white matter degenerate and are lost, leading to some disconnections between various parts of the central nervous system. Other myelinated nerve fibers are affected differently, because only their sheaths degenerate, leaving the axons intact. Such axons are remyelinated by a series of internodes that are much shorter than the original ones and are composed of thinner sheaths. Thus the myelin-forming cells of the central nervous system, the oligodendrocytes, remain active during aging. Indeed, not only do these neuroglial cell remyelinate axons, with age they also continue to add lamellae to the myelin sheaths of intact nerve fibers, so that sheaths become thicker. It is presumed that the degeneration of myelin sheaths is due to the degeneration of the parent oligodendrocyte, and that the production of increased numbers of internodes as a consequence of remyelination requires additional oligodendrocytes. Whether there is a turnover of oligodendrocytes during life has not been studied in primates, but it has been established that over the life span of the monkey, there is a substantial increase in the numbers of oligodendrocytes. While the loss of some myelinated nerve fibers leads to some disconnections, the degeneration of other myelin sheaths and the subsequent remyelination of axons by shorter internodes slow down the rate conduction along nerve fibers. These changes affect the integrity and timing in neuronal circuits, and there is evidence that they contribute to cognitive decline.

Distribution and progression of amyloid-beta deposits in the amygdala of the aged macaque monkey, and parallels with zinc distribution

In this study, we have mapped amyloid beta (Abeta) deposition in the amygdala of five aged Japanese monkeys (from 23 to 30 years old). In brief, the aged monkey amygdala shows a topographic distribution of Abeta deposits that is subnucleus specific and exhibits a distinct temporal progression. The pattern is similar to the distribution of Abeta deposits in the human amygdala of Alzheimer's patients and of high plaque nondemented cases. The spatial distribution and temporal progression were correlated with the distribution of free zinc (Zn), which is known to mediate Abeta aggregation. For the basolateral group of subnuclei in particular, there is a clear dorsoventral gradient in the progressive distribution of Abeta. Abeta depositions first appear in the ventral division of the lateral nucleus and parvicellular division of the accessory basal nucleus, and then extend into the ventral part of the basal and paralaminar nuclei. All these nuclei are also Zn-dense. Conversely, Zn-weak nuclei, which are more dorsally situated (i.e. dorsal division of lateral nucleus and magnocellular division of basal nucleus) showed only a low level of Abeta deposits, even in brains with the greatest Abeta burden. In contrast to the basolateral group, the central and medial nuclei and cortical group had Abeta deposits only at later stages. In the central and medial nuclei, we identified a lateromedial gradient of Abeta deposits, again similar to the gradient of Zn-distribution. In the cortical group, Abeta deposits are densest in the deep layer, where Zn is also densest. Thus, we suggest the macaque amygdala, with its clear topographic distribution of Abeta deposits, may be an effective model for examining the complex mechanisms of vulnerability to Abeta deposits. A primate model would be advantageous for experimental interventions geared toward therapeutic protection from Alzheimer's disease, including by microarray analysis and genetic manipulation.

Mechanisms of amyloid plaque pathogenesis

The first ultrastructural investigations of Alzheimer's disease noted the prominence of degenerating mitochondria in the dystrophic neurites of amyloid plaques, and speculated that this degeneration might be a major contributor to plaque pathogenesis. However, the fate of these organelles has received scant consideration in the intervening decades. A number of hypotheses for the formation and progression of amyloid plaques have since been suggested, including glial secretion of amyloid, somal and synaptic secretion of amyloid-beta protein from neurons, and endosomal-lysosomal aggregation of amyloid-beta protein in the cell bodies of neurons, but none of these hypotheses fully account for the focal accumulation of amyloid in plaques. In addition to Alzheimer's disease, amyloid plaques occur in a variety of conditions, and these conditions are all accompanied by dystrophic neurites characteristic of disrupted axonal transport. The disruption of axonal transport results in the autophagocytosis of mitochondria without normal lysosomal degradation, and recent evidence from aging, traumatic injury, Alzheimer's disease and transgenic mice models of Alzheimer's disease, suggests that the degeneration of these autophagosomes may lead to amyloid production within dystrophic neurites. The theory of amyloid plaque pathogenesis has thus come full circle, back to the intuitions of the very first researchers in the field.

Dysfunction and death of neurons in human degenerative neurological diseases and in animal models

The human neurological disorders--amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), and Alzheimer's disease (AD)--share certain features: they occur in later stages of adult life; are slowly progressive; and involve specific groups of nerve cells. Different clinical syndromes result from dysfunction and death of these specific groups of neurons. In ALS, patients are weak due to disease of motor neurons in the spinal cord. The clinical features of PD, e.g. slow movements, tremor and rigidity, are attributed, in part, to degeneration of dopaminergic neurons of the substantia nigra. Impairments of cognition and memory in AD result from disease of neurons in a number of regions, including brainstem, basal forebrain, amygdala, hippocampus, and neocortex. In each of these diseases, affected neurons exhibit abnormalities of the neuronal cytoskeleton: in ALS, neurofilaments accumulate and distend proximal motor axons; in PD, nigral perikarya show Lewy bodies-intracytoplasmic inclusions containing neurofilament antigens; in AD, neurons develop neurofibrillary tangles, Hirano bodies, granulovacuolar degeneration and filament-filled neurites in plaques. Certain features of ALS, PD and AD are recapitulated in animal models, three of which are described in this review. Hereditary canine spinal muscular atrophy (HCSMA), a dominantly inherited motor neuron disease, shows many clinical and pathological features in common with ALS, including weakness, muscle atrophy, neurofilamentous swellings of proximal axons, impaired transport of neurofilament proteins, and degeneration of motor neurons. In primates, intoxication with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces a parkinsonian syndrome due to injury of nigral dopaminergic neurons and associated denervation of the striatum. Finally, aged macaques exhibit memory deficits, and their cerebral cortices show senile plaques and filament-filled neurites derived from a variety of transmitter-specific populations of nerve cells. In human diseases, the causes and mechanisms leading to dysfunction and death of nerve cells are unknown. Investigators have begun using a variety of techniques derived from neurobiology to study animal models in an effort to clarify the mechanisms, evolutions, and consequences of structural-chemical abnormalities occurring in different neuronal systems implicated in human disease. Understanding such processes in these models should provide important new insights into the pathogeneses of similar processes occurring in ALS, PD and AD.

Mitochondrial degeneration in dystrophic neurites of senile plaques may lead to extracellular deposition of fine filaments

Recent data show that amyloid precursor protein accumulates inside axons after disruption of fast axonal transport, but how this leads to mature plaques with extracellular amyloid remains unclear. To investigate this issue, primitive plaques in prefrontal cortex of aged rhesus monkeys were reconstructed using serial section electron microscopy. The swollen profiles of dystrophic neurites were found to be diverticula from the main axis of otherwise normal neurites. Microtubules extended from the main neurite axis into the diverticulum to form circular loops or coils, providing a transport pathway for trapping organelles. The quantity and morphology of organelles contained within diverticula suggested a progression of degeneration. Primitive diverticula contained microtubules and normal mitochondria, while larger, presumably older, diverticula contained large numbers of degenerating mitochondria. In advanced stages of degeneration, apparent autophagosomes derived from mitochondria exhibited a loose lamellar to filamentous internal structure. Similar filamentous material and remnants of mitochondria were visible in the extracellular spaces of plaques. This progression of degeneration suggests that extracellular filaments originate inside degenerating mitochondria of neuritic diverticula, which may be a common process in diverse diseases.

Rodent A beta modulates the solubility and distribution of amyloid deposits in transgenic mice

The amino acid sequence of amyloid precursor protein (APP) is highly conserved, and age-related A beta aggregates have been described in a variety of vertebrate animals, with the notable exception of mice and rats. Three amino acid substitutions distinguish mouse and human A beta that might contribute to their differing properties in vivo. To examine the amyloidogenic potential of mouse A beta, we studied several lines of transgenic mice overexpressing wild-type mouse amyloid precursor protein (moAPP) either alone or in conjunction with mutant PS1 (PS1dE9). Neither overexpression of moAPP alone nor co-expression with PS1dE9 caused mice to develop Alzheimer-type amyloid pathology by 24 months of age. We further tested whether mouse A beta could accelerate the deposition of human A beta by crossing the moAPP transgenic mice to a bigenic line expressing human APPswe with PS1dE9. The triple transgenic animals (moAPP x APPswe/PS1dE9) produced 20% more A beta but formed amyloid deposits no faster and to no greater extent than APPswe/PS1dE9 siblings. Instead, the additional mouse A beta increased the detergent solubility of accumulated amyloid and exacerbated amyloid deposition in the vasculature. These findings suggest that, although mouse A beta does not influence the rate of amyloid formation, the incorporation of A beta peptides with differing sequences alters the solubility and localization of the resulting aggregates.

What's behind the decline? The role of white matter in brain aging

The specific molecular events that underlie the age-related loss of cognitive function are poorly understood. Although not experimentally substantiated, age-dependent neuronal loss has long been considered central to age-related cognitive decline. More recently, age-related changes in brain white matter have taken precedence in explaining the steady decline in cognitive domains seen in non-diseased elderly. Characteristic alterations in the ultrastructure of myelin coupled with evidence of inflammatory processes present in the white matter of several different species suggest that specific molecular events within brain white matter may better explain observed pathological changes and cognitive deficits. This review focuses on recent evidence highlighting the importance of white matter in deciphering the course of "normal" brain aging.

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.

Frontal lobe volume, function, and beta-amyloid pathology in a canine model of aging

Application of magnetic resonance imaging (MRI) techniques reveals that human brain aging varies across cortical regions. One area particularly sensitive to normal aging is the frontal lobes. In vitro neuropathological studies and behavioral measures in a canine model of aging previously suggested that the frontal lobes of the dog might be sensitive to aging. In the present study, MRI scans were acquired to compare age-related changes in frontal lobe volume with changes in executive functions and beta-amyloid pathology in the frontal cortex of beagle dogs aged 3 months to 15 years. Decreases in total brain volume appeared only in senior dogs (aged 12 years and older), whereas frontal lobe atrophy developed earlier, appearing in the old dogs (aged 8-11 years). Hippocampal volume also declined with age, but not occipital lobe volume past maturity. Reduced frontal lobe volume correlated with impaired performance on measures of executive function, including inhibitory control and complex working memory, and with increased beta-amyloid accumulation in the frontal cortex. Age-related hippocampal atrophy also correlated with complex working memory but not inhibitory control, whereas occipital lobe volume did not correlate with any cognitive measure. These findings are consistent with the frontal lobe theory of aging in humans, which suggests that the frontal lobes and functions subserved by this region are compromised early in aging.

The role of executive functioning in CBT: a pilot study with anxious older adults

Cognitive behavior therapy (CBT) is an effective treatment for late life anxiety and depression. The successful use of CBT is assumed to rely on cognitive skills known as executive functions (EF; e.g., hypothesis generation, allocation of attention, self-monitoring) governed by the prefrontal cortex. Because older adults sometimes have executive deficits as a consequence of normal aging, EF may be a mediator of CBT outcome in older samples. The current pilot study tested the hypothesis that older adults with executive deficits (as measured by neuropsychological tests) would show decreased therapeutic benefit from CBT for generalized anxiety disorder, as compared to a group with intact EF. Results indicated differential response to CBT within the dysfunction group depending on the stability (and possibly, the etiology) of executive deficits from pre- to posttreatment. Those whose EF scores remained low from pre- to posttreatment did not respond to CBT, while those whose scores improved responded quite well, similar to an Intact EF group. Results indicate that some, but not all, older adults with executive dysfunction show decreased benefit from CBT, and are consistent with the assumption that executive skills are important for the successful use of CBT. However, some participants may show improvement on both mood and cognitive skills during treatment, which is discussed further.

Alzheimer's disease abeta vaccine reduces central nervous system abeta levels in a non-human primate, the Caribbean vervet

Amyloid beta (Abeta) protein immunotherapy lowers cerebral Abeta and improves cognition in mouse models of Alzheimer's disease (AD). Here we show that Caribbean vervet monkeys (Chlorocebus aethiops, SK) develop cerebral Abeta plaques with aging and that these deposits are associated with gliosis and neuritic dystrophy. Five aged vervets were immunized with Abeta peptide over 10 months. Plasma and cerebral spinal fluid (CSF) samples were collected periodically from the immunized vervets and five aged controls; one monkey per group expired during the study. By Day 42, immunized animals generated plasma Abeta antibodies that labeled Abeta plaques in human, AD transgenic mouse and vervet brains; bound Abeta1-7; and recognized monomeric and oligomeric Abeta but not full-length amyloid precursor protein nor its C-terminal fragments. Low anti-Abeta titers were detected in CSF. Abetax-40 levels were elevated approximately 2- to 5-fold in plasma and decreased up to 64% in CSF in immunized vervets. Insoluble Abetax-42 was decreased by 66% in brain homogenates of the four immunized animals compared to archival tissues from 13 age-matched control vervets. Abeta42-immunoreactive plaques were detected in frontal cortex in 11 of the 13 control animals, but not in six brain regions examined in each of the four immunized vervets. No T cell response or inflammation was observed. Our study is the first to demonstrate age-related Abeta deposition in the vervet monkey as well as the lowering of cerebral Abeta by Abeta vaccination in a non-human primate. The findings further support Abeta immunotherapy as a potential prevention and treatment of AD.

Beta-secretase activity increases with aging in human, monkey, and mouse brain

Amyloid beta protein (A beta) accumulates in the brains of aging humans, amyloid precursor protein (APP) transgenic mouse lines, and rhesus monkeys. We tested the hypothesis that aging was associated with increased activity of the beta-site amyloid precursor protein cleaving enzyme (beta-secretase, BACE) in brain. We evaluated BACE activity, BACE protein, and formic acid-extractable A beta levels in cohorts of young (4 months old) and old (14 to 18 months old) nontransgenic mice (n = 16) and Tg2576 APP transgenic mice (n = 17), young (4.4 to 12.7 years old) and old (20.9 to 30.4 years old) rhesus monkeys (n = 17), and a wide age range (18 to 92 years old) of nondemented human brains (n = 25). Aging was associated with increased brain A beta levels in each cohort. Furthermore BACE activity increased significantly with age in mouse, monkey, and human brains, independent of brain region. BACE protein levels, however, were unchanged with age. BACE activity correlated with formic acid-extractable A beta levels in transgenic mouse, nontransgenic mouse, and human cortex, but not in monkey brain. These data suggest that an age-related increase of BACE activity contributes to the increased production and accumulation of brain A beta, and potentially predisposes to Alzheimer's disease in humans.

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.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder, characterized by a progressive loss of cognitive function. Despite considerable progress, a complete description of the molecular pathology of this disease has yet to be elucidated. In this respect, the need for an animal model that develops some or all aspects of this uniquely human disease in a reproducible fashion is crucial for the development and testing of potential treatments. A valid animal model for AD should exhibit (1) progressive AD-like neuropathology and (2) cognitive deficits, and (3) should be verified in several laboratories. Transgenic models should be able to (4) discern pathogenic effects of familial forms (FAD) mutations from those of transgene overexpression. Models derived from microinjection of FAD mutant alleles should (5) encompass more than one Tg line. At present, however, no model that replicates all of these desirable features exists. In this review, we discuss transgenic mouse models with well-characterized AD-like neuropathology that show some form of cognitive impairment. We argue that conclusions drawn from a limited selection of cross-sectional experiments should be verified in longitudinally designed experiments. Future studies should attempt to establish a closer relationship between molecular pathology and the degree of cognitive impairment. While exact replication of AD in mice may not attainable (due to phylogenetic differences and fundamental differences in behavioral ecology), rigorous comparative analysis of cognitive behavior observed in various mouse models of AD should provide a framework for better understanding of molecular mechanisms underlying cognitive impairment observed in AD patients.

Senile plaques in an aged western lowland gorilla

Senile plaques (SPs) were found in the cerebral cortex of a 44-year-old Western lowland gorilla (Gorilla gorilla gorilla). All the SPs were obtained as dense assemblies consisting of fibrous materials by silver impregnation, but were not detected by Congo red. More SPs were detected by immunostaining for amyloid beta protein (A beta) and a half of A beta-positive-SPs were also immunoreactive for apolipoprotein E. Moreover, all SPs were immunoreactive for A beta 42 and A beta 43, but not for A beta 40. SPs also did not contain A beta precursor protein-positive structures. These findings suggested that SPs in this case were diffuse plaques. To our knowledge, this is the first report of SPs in the gorilla.

Conservation of neuron number and size in entorhinal cortex layers II, III, and V/VI of aged primates

Past dogma asserted that extensive loss of cortical neurons accompanies normal aging. However, recent stereologic studies in humans, monkeys, and rodents have found little evidence of age-related neuronal loss in several cortical regions, including the neocortex and hippocampus. Yet to date, a complete investigation of age-related neuronal loss or size change has not been undertaken in the entorhinal cortex, a retrohippocampal structure essential for learning and memory. The aged rhesus macaque monkey (Macaca mulatta), a species that develops beta-amyloid plaques and exhibits cognitive deficits with age, is considered the best commonly available model of aging in humans. In the present study, we examined changes in total neuron number and size in layers II, III, and V/VI of the intermediate division of the entorhinal cortex in aged vs. nonaged rhesus monkeys by using unbiased stereologic methods. Total neuron number was conserved in aged primates when compared with nonaged adults in entorhinal cortex layer II (aged = 56,500 +/- 12,100, nonaged adult = 48,500 +/- 10,900; P = 0.37), layer III (aged = 205, 600 +/- 50,700, nonaged adult = 187,600 +/- 60,300; P = 0.66), and layers V/VI (aged = 246,400 +/- 76,700, nonaged adult = 236,800 +/- 69,600; P = 0.87). In each of the layers examined, neuronal area and volume were also conserved with aging. This lack of morphologically evident neurodegeneration in primate entorhinal cortex with aging further supports the concept that fundamental differences exist between the processes of normal "healthy" aging and pathologic age-related neurodegenerative disorders such as Alzheimer's disease.

Immunohistochemical analysis of cerebral cortical and vascular lesions in the primate Microcebus murinus reveal distinct amyloid beta1-42 and beta1-40 immunoreactivity profiles

Recent reports have shown that amyloid beta deposits in the brains of Alzheimer's disease patients consist mainly of two distinct species of amyloid beta protein (Abeta) with different C-termini, Abeta1-42 (Abeta42) and Abeta1-40 (Abeta40). The nature of the Abeta species in Microcebus murinus brain was investigated immunocytochemically using polyclonal antibodies with clear specificity for the Abeta42 and Abeta40 C-termini. The cortical vascular deposits were immunopositive for both Abeta42 and Abeta40. However, most of the diffuse plaques were strongly positive for Abeta42 whereas only a subset of deposits were positive for Abeta40. Numerous cortical plaques were Abeta42-immunopositive but tested negative for Abeta40. This suggests that Abeta42 is probably associated with early stages of plaque maturation. This neuropathological feature reminiscent of that observed in brains affected by Alzheimer's disease further supports the idea that M. murinus could be used as a potential model of the early stages of this neurological disease.

Filamentous tau pathology in nerve cells, astrocytes, and oligodendrocytes of aged baboons

Intracellular filamentous inclusions containing abnormally phosphorylated tau protein are hallmarks of several human neurodegenerative disorders. This study reveals tau-positive cytoskeletal abnormalities in neurons and glial cells of aged baboons. The brains of four baboons (Papio hamadryas, 20-30 yr of age) were examined using the Gallyas silver technique for neurofibrillary changes and phosphorylation-dependent anti-tau antibodies (AT8, AT100, AT270, PHF-1, TG-3). Conspicuous changes were noted in two animals, 26 and 30 yr of age. In both animals, a combination of neuronal and glial cytoskeletal pathology was seen preferentially affecting limbic brain areas, including the hippocampal formation. In the 30-yr-old animal, numerous tau-positive inclusions were seen in the granule cells of the fascia dentata. These cells even exhibited an accumulation of argyrophilic neurofibrillary tangles. The glial changes affected both astrocytes and oligodendrocytes. Tau-positive astrocytes were seen in perivascular, subpial, and subependymal locations. Tau-positive oligodendrocytes preferentially occurred in limbic fiber tracts including the entorhinal perforant path. Ultrastructurally, tau-positive straight filaments (10-14 nm) in both neurons and glial cells were revealed by anti-tau immunoelectron microscopy. This study thus indicates the potential usefulness of aged baboons for experimental investigation of neuronal and glial filamentous tau pathology. This nonhuman primate species may provide valuable information pertinent to the broad spectrum of human tauopathies.

MRI description of cerebral atrophy in mouse lemur primates

We assessed cerebral atrophy in mouse lemur primates (Microcebus murinus) by estimating CSF volume in their brains from 4.7 Tesla T2-weighted magnetic resonance images. Thirty animals aged from 1 to 10.3 years were imaged, 14 of them were followed for up to 2 years. Seven of these animals were examined for neuropathology. In 12 out of 17 animals older than 3.5 years, CSF volumes were increased. A subgroup of six animals had severe atrophy of the temporal lobe. Another subgroup of five animals displayed diffuse atrophy in addition to the temporal atrophy. One animal had a dilation of the external part of the temporal horn of the lateral ventricle in addition to the temporal atrophy. The three animals with diffuse atrophy that could be studied for neuropathology had diffuse cerebral amyloid deposits detected by immunocytochemistry. The other animals did not display amyloid deposits. Relations between the different types of atrophy as well as their causes will have to be assessed in future studies.

Histopathological studies of senile plaques and cerebral amyloidosis in cynomolgus monkeys

Senile plaques (SPs) and cerebral amyloid angiopathy (CAA), pathological hallmarks of Alzheimer's disease, have not been thoroughly investigated histopathologically in nonhuman primates. To determine the onset age and histopathological characteristics of SPs and CAA, we examined the brains of 64 cynomolgus monkeys (Macaca fascicularis) from 2 to 35 years old. Mature (classical and primitive) plaques appeared in 16 out of 25 monkeys that were >20 years old. Moreover, mature plaques were observed more frequently than diffuse plaques and were located in the temporal cortex of the superior or inferior gyri and amygdala. Diffuse plaques in contrast to mature plaques did not show definite tendencies in onset age and distribution. CAA appeared in more than 22-year-old monkeys in 10 out of 16 animals and was frequently observed in capillaries and often found adjoining mature plaques. During immunohistochemical examination, an antiserum for amyloid beta protein (A beta) 1-40 could detect all SPs, whereas a monoclonal antibody for A beta 8-17 could not detect any diffuse plaques and only one third of the primitive plaques. As for CAA, the polyclonal antiserum was more sensitive than the monoclonal antibody. The present study describes the histopathological features of SPs and CAA in old cynomolgus monkeys.

Visual-discrimination learning ability and beta-amyloid accumulation in the dog

Young, middle-aged, and old beagle dogs were tested on several visual-discrimination tasks: reward- and object-approach learning, object discrimination and reversal, long-term retention of a reversal problem, and a size-discrimination task. Beta-amyloid accumulation in the entorhinal, prefrontal, parietal, and occipital cortices was quantified using immunohistochemical and imaging techniques at the conclusion of cognitive testing. Middle-aged and old dogs were impaired in size-discrimination learning. In each task, a subset of aged dogs was impaired relative to age-matched peers. Beta-amyloid accumulation was age-dependent. However, not all middle-aged and old dogs showed beta-amyloid accumulation in the entorhinal cortex. The error scores from dogs tested with a nonpreferred object during visual discrimination learning and from reversal learning were correlated with beta-amyloid in the prefrontal but not entorhinal cortex. Size-discrimination and reward and object-approach learning error scores were correlated with beta-amyloid accumulation in the entorhinal but not prefrontal cortex. The results of these studies support an association between cognitive test and the location and extent of beta-amyloid pathology.

Symposium overview: the use of delayed matching-to-sample procedures in studies of short-term memory in animals and humans

Behavioral paradigms applicable for use in both human and nonhuman subjects for investigating aspects of working/short-term memory are presented with a view towards exploring their strengths, weaknesses, and utility in a variety of experimental situations. Such procedures can be useful in teasing out specific aspects of mnemonic processes including discrimination, encoding, and retention. Delayed matching-to-position, delayed matching-to-sample (DMTS), and titrating matching-to-sample procedures are highlighted. Additionally, the application of DMTS tasks in preclinical and clinical settings is presented: drug effects on memory processes can be explored preclinically in animal models; normative data have been developed in human populations where they have been used in adults to explore the relationships between mnemonic processes and specific clinical entities such as Parkinsonism, senile dementia of the Alzheimer's type, schizophrenia, and depression. Studies in children indicate that encoding and retention processes improve rapidly in the early years, plateauing prior to puberty. Noninvasive imaging techniques such as positron emission tomography (PET) indicate that activity in specific brain areas is associated with DMTS task performance and may serve to confirm roles for such structures in mnemonic processes.

Aging renders the brain vulnerable to amyloid beta-protein neurotoxicity

The formation of fibrillar deposits of amyloid beta protein (Abeta) in the brain is a pathological hallmark of Alzheimer's disease (AD). A central question is whether Abeta plays a direct role in the neurodegenerative process in AD. The involvement of Abeta in the neurodegenerative process is suggested by the neurotoxicity of the fibrillar form of Abeta in vitro. However, mice transgenic for the Abeta precursor protein that develop amyloid deposits in the brain do not show the degree of neuronal loss or tau phosphorylation found in AD. Here we show that microinjection of plaque-equivalent concentrations of fibrillar, but not soluble, Abeta in the aged rhesus monkey cerebral cortex results in profound neuronal loss, tau phosphorylation and microglial proliferation. Fibrillar Abeta at plaque-equivalent concentrations is not toxic in the young adult rhesus brain. Abeta toxicity in vivo is also highly species-specific; toxicity is greater in aged rhesus monkeys than in aged marmoset monkeys, and is not significant in aged rats. These results suggest that Abeta neurotoxicity in vivo is a pathological response of the aging brain, which is most pronounced in higher order primates. Thus, longevity may contribute to the unique susceptibility of humans to Alzheimer's disease by rendering the brain vulnerable to Abeta neurotoxicity.

Expressing mRNAs for presenilin-1 and amyloid precursor protein (APP-695) from same neuronal populations in rat hippocampus

Presenilin-1 gene (PS1, s182) encoding a protein that contains seven transmembrane domains was recently identified as a risk factor for Alzheimer's disease (AD). Mutations on this gene may contribute to pathogenesis of AD. In this study, we used in situ hybridization histochemistry to detect the expression of PS1 in neurons in the hippocampal formation and cerebral cortex of the rat brain. Specifically, mRNAs of PS1 were predominantly found in pyramidal neurons in CA1, CA3, and granule cells in the hilar region of the dentate gyrus of the hippocampus and in the external and internal pyramidal layers of the cerebral cortex. The same neuronal populations express the amyloid protein precursor (APP-695). Transgenic mice expressing a mutated APP-695 produced increased levels of beta-amyloid (A beta) and exhibited behavioral deficits. These results lead us to suggest that when PS1 and APP-695 colocalize in neurons in the brain, PS1 may play a regulatory role in production of (A beta) protein of AD.

Cerebral glucose metabolism and memory in aged rhesus macaques

Positron emission tomography and the glucose metabolic tracer [18F]fluorodeoxyglucose were used to evaluate the relationship between regional cerebral metabolic rates for glucose (rCMRglc), age, and performance on a delayed response (DR) test of memory in the aged monkey. Eleven aged animals, 21-26-years old, were included in the analysis. Regional CMRglc, normalized to values for the entire brain, were determined for the dorsal prefrontal cortex, orbitofrontal cortex, hippocampus, and temporal cortex. The aged animals exhibited significant DR deficits relative to a cohort of normal young monkeys. Variability in DR performance among the aged subjects was significantly correlated with relative hippocampal rCMRglc, and chronological age was a reliable predictor of orbitofrontal rCMRglc ratios. This pattern of results suggests that DR impairments in the aged monkey may partly reflect age-related dysfunction distributed among multiple limbic system structures that participate in normal learning and memory. Overall, the findings support the use of positron emission tomography in efforts to define the relationship between cognitive performance, age, and brain physiology in nonhuman primates.

Co-localization of beta-amyloid peptides, apolipoprotein E and glial markers in senile plaques in the prefrontal cortex of old rhesus monkeys

Based on the homology of human and monkey amyloid precursor proteins and the derived beta-amyloid peptides (A beta) the investigation of brains from old monkeys might be useful for the understanding of beta-amyloidosis in the aetiology of Alzheimer's disease. In the present study, the prefrontal cortex, which is known to be highly susceptible to the deposition of A beta, was screened for the occurrence of senile plaques in perfused tissue of aged rhesus monkeys (Macaca mulatta). A beta deposits were immunocytochemically detected in five of six macaques aged about 28 years. Differently N-terminal truncated A beta species in the senile plaques were simultaneously detected by a carbocyanine double fluorescence method applying the bright red fluorescent Cy3 and the novel green fluorescent Cy2. In a few cases, immunoreactivity for the shortened fragment containing the amino acids 17-42 (A beta(17-42); p3 fragment with a molecular weight of 3 kDa) was demonstrated in deposits apparently devoid of A beta(8-17). Senile plaques were further characterized by carbocyanine double labelling of A beta and astrocytes, microglia and apolipoprotein E.

Age-related pathology and biosenescent markers in captive rhesus macaques

During the past 15 years, our aging colony of rhesus monkeys, consisting of animals from 20 to 37 years of age, had an annual average population of 88.2 live monkeys and, of this population, an annual average of 13.9 monkeys died spontaneously or were terminated due to severe illness. From 1980 to 1994, a total of 175 autopsies of rhesus macaques, from 20 to 37 years of age, were performed. By cumulative autopsy data, the incidence of age-related pathology in various organs was surveyed. Major geriatric diseases such as coronary sclerosis, emphysema, degenerative joint disorders, cancer, and cerebral amyloid plaque began to develop in 10 to 40% of macaques after 20 years and the incidence significantly increased after 26 years of age. Approximately 12% of aged macaques from 20 to 30 years of age died annually due to such geriatric diseases with severe complications. The average survival rate indicated that half the population at 20 years of age died by 25 years and 73% died by 30 years of age. Less than 10% of macaques survived over 30 years. Using these aged macaques as well as other juvenile to adult monkeys in our Center, clinical opththalmological and reproductive endocrinological studies, and magnetic resonance imaging (MRI) of the brain were conducted to define bioaging markers of captive rhesus monkeys. Cataracts began to develop in 20% of rhesus monkeys at 20 to 22 years of age and the rate significantly increased after 26 years of age. Menopause occurred at 26 to 27 years of age. Multiple cerebral infarctions and iron deposits in the globus pallidus and substantia nigra were detected by MRI in the aged brains. These geriatric disorders in captive aged macaques appear to be natural aging outcomes, since the simple lifestyle of these captive animals offers minimal exposure to environmental factors. Our data will offer useful paradigms for preventive or experimental studies on age-related diseases.