Senile plaques in cortex of aged normal monkeys

Alpha 1-antichymotrypsin is present together with the beta-protein in monkey brain amyloid deposits

The recent finding that the serine protease inhibitor, alpha 1-antichymotrypsin, is tightly associated with the amyloid deposits in brains of normal aged individuals and patients with Alzheimer's disease [Abraham C. R., Selkoe D. J. and Potter H. (1988) Cell 52, 487-501], suggests a role for this inhibitor in the progressive deposition of brain amyloid in humans. We have used immunocytochemistry to detect alpha 1-antichymotrypsin in the amyloid that accumulates in brains of aged monkeys, a naturally occurring animal model of Alzheimer-like neuropathology. In monkeys of increasing age, the earliest alpha 1-antichymotrypsin immunoreactivity was found in cortical perivascular cells, before the appearance of either Thioflavin S-detectable amyloid deposits or beta-protein reactivity in the vessel walls. Subsequently, amyloid deposits appeared in small meningeal blood vessels and cortical neuritic plaques. The oldest monkeys also showed microvascular amyloid in the cortical gray matter. Amyloid was never seen in white matter. The amyloid deposits in meningeal vessels were always positive for both beta-protein and alpha 1-antichymotrypsin, whereas in the cortex, alpha 1-antichymotrypsin immunoreactivity seemed to appear somewhat later than that of beta-protein. These findings demonstrate that two of the brain amyloid components of human senescence and Alzheimer's disease--the beta-protein and the protease inhibitor alpha 1-antichymotrypsin--are also present in the amyloid deposits of normal aged monkey brain. The extended molecular parallels between normal brain aging and Alzheimer's disease suggest that similar biochemical mechanisms may underlie progressive amyloid deposition in both situations.

Age-related changes in multiple neurotransmitter systems in the monkey brain

Age-associated changes in cholinergic, monoaminergic and amino acid neurotransmitter systems were analyzed in 14 brain regions of 23 rhesus monkeys that ranged in age from 2 to 37 years. In the frontal pole, the levels of choline acetyltransferase (ChAT) activity, the density of [3H]ketanserin (serotonin type-2) binding sites and endogenous levels of dopamine, homovanillic acid and serotonin, all expressed per milligram of protein, decreased significantly with aging. In precentral motor cortex, ChAT activity decreased; in parietal and occipital cortex, the number of [3H]ketanserin binding sites decreased while the number of Na+-independent [3H]glutamate binding sites increased with age. In the caudate nucleus, endogenous levels of norepinephrine decreased. This descriptive study indicates that the aging monkey may be a very useful model for future investigations of age-associated transmitter abnormalities similar to those that occur in humans.

The protease inhibitor, alpha 1-antichymotrypsin, is a component of the brain amyloid deposits in normal aging and Alzheimer's disease

The purpose of this study was to characterize the nature and the origin of the Alzheimer's disease amyloid deposits. We used an amyloid antiserum to screen a human liver expression library. A positive clone was sequenced and found to code for the serine protease inhibitor alpha 1-antichymotrypsin, an acute phase serum protein. Thus, this protein is a second component of the brain amyloid in addition to the beta-protein. In order to determine whether the inhibitor originated from the serum or was made in the brain, we performed Northern blots on tissue from control and Alzheimer brain and found that alpha 1-antichymotrypsin RNA is present in the brain and that the diseased brain contained larger amounts than the controls. Immunocytochemistry and in situ hybridization show the astrocytes to produce the inhibitor, mainly around senile plaques, alpha 1-antichymotrypsin is only associated with the amyloid deposits of the beta-protein kind in normal aging of man and monkeys. Alzheimer's, Down's syndrome and hereditary cerebral hemorrhage with amyloidosis of Dutch origin, but not in primary and secondary amyloidosis or familial amyloidotic polyneuropathy. The specific association between alpha 1-antichymotrypsin and the beta-protein prompted us to suggest a role for this serine protease inhibitor in the proteolytic processing of the beta-protein precursor.

Serotoninergic neurites in senile plaques in cingulate cortex of aged nonhuman primate

In immunocytochemical studies, a polyclonal antiserotonin antibody was used to visualize fibers within the cingulate cortex of young and aged rhesus monkeys. Intricate and distinct patterns of serotoninergic processes were seen in anterior and posterior segments of cingulate cortex (Brodmann areas 24 and 23). In these regions of cortex, many multivaricose serotonin-immunoreactive axonal swellings were identified, and some of these immunostained neurites were associated with deposits of amyloid. These observations suggest that serotoninergic processes are involved in the formation of senile plaques in neocortex of aged macaques.

The neural basis of memory decline in aged monkeys

Nonhuman primates experience changes in behavior as they progress into old age. Visual recognition, spatial learning, habit formation, and visuospatial manipulation are impaired in aged rhesus monkeys relative to young controls. We have begun to study the possible neural substrate for these changes, focusing on brain areas that are known, from lesion studies, to be essential for the successful performance of specific tasks. Aged nonhuman primates develop senile plaques, most commonly in amygdala, hippocampus, and neocortex. Our preliminary data suggest that the density of plaques may be related to poor behavioral performance in some aged monkeys. However, behavioral decline begins before the appearance of significant numbers of senile plaques, suggesting that other factors may interfere with cognition. Numerous studies of several genera have shown that receptors for neurotransmitters decline in number between the adolescent years and old age. Our autoradiographic analyses of primate temporal neocortex demonstrate loss of muscarinic, nicotinic, dopaminergic and serotoninergic receptor binding sites between the ages of 2 and 22 years. Preliminary data indicate that markers for adenyl cyclase and phosphatidyl inositol second-messenger systems also are reduced in temporal cortex. Although these declines represent a potential substrate for behavioral changes, no studies have directly related a decrease in receptor number to deficits in learning and memory in aged primates. Other changes in the aging brain include loss of neurons, reduced neurochemical markers, and decreased content of neuronal ribonucleic acid (RNA). All of these decrements may be interrelated to some extent in that decreased RNA could result in changes in neurochemical markers and receptors and, eventually, in dysfunction and death of neurons. These observations underscore the importance of establishing a time course for age-associated neural abnormalities, examining regions of brain in which changes are most likely to occur, and studying their relationship to the progression of behavioral dysfunction. Detailed anatomical analyses of the distribution of in situ uptake/receptor binding sites and messenger RNA (mRNA) in aged nonhuman primates may clarify some of the factors that most likely contribute to behavioral changes in elderly humans.

Effects of aging on visual recognition memory in the rhesus monkey

As part of an effort to develop a primate model of human age-related memory dysfunction, performance by six rhesus monkeys 26 to 27 years of age was compared to that of six young adult monkeys (four to five years of age) on a trial unique delayed nonmatching to sample (DNMS) task. This task assesses the monkey's ability to identify a novel from a familiar stimulus over a delay and resembles closely clinical tests that are used to assess memory function in geriatric patients. The task was presented in three stages: acquisition, delays and lists. As a group, aged monkeys were impaired relative to the young adult group on all three conditions. However, within the aged group, individual cases of efficient performance were observed. Error analyses of item positions of the lists condition revealed the absence of enhanced performance for items presented at the end of a list by aged animals, suggesting an abnormal sensitivity to proactive interference. The finding of a recognition impairment with age is in parallel with studies of normal human aging and lends support to the notion that the rhesus monkey is a suitable animal model of human aging.

Absence of mutation in the beta-amyloid cDNAs cloned from the brains of three patients with sporadic Alzheimer's disease

Using an oligonucleotide probe, we isolated cDNA clones corresponding to the precursor of the beta-amyloid peptide (BAP) from brain libraries of 3 patients with sporadic Alzheimer's disease (AD). DNA sequencing showed that the largest cDNA clone encompasses 83% of the open reading frame proposed by Kang et al. to encode the BAP precursor (APP). cDNA clones from each of the 3 AD brain libraries were identical to the sequence of the APP-cDNAs cloned from normal adult human and fetal brain. An antisense-radiolabeled RNA copy of one of the AD clones detected a pattern of 3 gene transcripts measuring 3.5, 3.2 and 1.6 kilobases (kb) in both normal and AD brain RNAs. These data suggest that there are no mutations in or about the 42 amino acid (aa) sequence of BAP and that the accumulation of amyloid consistently found in AD may result from altered post-translational processing of APP.

Tetrahydroaminoacridine, 3,4 diaminopyridine and physostigmine: direct comparison of effects on memory in aged primates

The effects of tetrahydroaminoacridine (THA), 3,4 diaminopyridine (3,4 DAP) and physostigmine were evaluated for their ability to reduce memory impairments in aged, test-sophisticated cebus monkeys (18 to 26 years old). Several doses of each drug were tested (PO) in each of ten different monkeys, allowing for direct and extensive comparison of each drug's efficacy in this model. The results of this comparative test revealed several potentially interesting findings: (1) all drugs produced improvement in a portion of the monkeys tested; (2) as in many past tests with aged monkeys and humans, wide variations in most effective dose, per subject, were observed; (3) different monkeys responded more effectively to one drug than another; and (4) under these tightly controlled conditions, physostigmine produced the most reliable and robust effects (p less than 0.005), in more monkeys, than did either THA (p less than 0.05) or 3,4 DAP (p less than 0.10).

Innervation of human hippocampus by noradrenergic systems: normal anatomy and structural abnormalities in aging and in Alzheimer's disease

Immunocytochemical studies, using an antibody directed against human dopamine beta-hydroxylase, identified an extensive plexus of noradrenergic axons/terminals in normal human hippocampus. In hippocampi of individuals with Alzheimer's disease, the density of noradrenergic innervation was reduced and abnormal noradrenergic axons, which exhibited multifocal enlargements, were present in the neuropil. Some of these neurites were clustered around deposits of amyloid (senile plaques), and these abnormalities were most common in CA3-4, a region normally showing a relatively high density of noradrenergic terminals. This investigation provides direct evidence for structural abnormalities of noradrenergic axons/nerve terminals in hippocampi of individuals with Alzheimer's disease.

Immunochemical identification of the serine protease inhibitor alpha 1-antichymotrypsin in the brain amyloid deposits of Alzheimer's disease

Two approaches--molecular cloning and immunochemical analysis--have identified one of the components of Alzheimer's disease amyloid deposits as the serine protease inhibitor alpha 1-antichymotrypsin. An antiserum against isolated Alzheimer amyloid deposits detected immunoreactivity in normal liver. The antiserum was then used to screen a liver cDNA expression library, yielding three related clones. DNA sequence analysis showed that these clones code for alpha 1-antichymotrypsin. Antisera against purified alpha 1-antichymotrypsin stained Alzheimer amyloid deposits, both in situ and after detergent extraction from brain. The anti-amyloid antiserum recognizes at least two distinct epitopes in alpha 1-antichymotrypsin, further supporting the presence of this protein in Alzheimer amyloid deposits. In addition to being produced in the liver and released into the serum, alpha 1-antichymotrypsin is expressed in Alzheimer brain, particularly in areas that develop amyloid lesions. Models by which alpha 1-antichymotrypsin could contribute to the development of Alzheimer amyloid deposits are discussed.

Age differences in recognition memory of the rhesus monkey (Macaca mulatta)

Aging is accompanied by a gradual decline in memory in both humans and nonhuman primates. To determine whether the impairment in nonhuman primates extends to recognition memory, which is a sensitive index of the integrity of the limbic system, we trained rhesus monkeys of four different age groups (3-6, 14-17, 20-24, and 25-29 years of age) on a delayed nonmatching-to-sample task with trial-unique objects. After the animals had learned the task, which required recognition of single objects presented ten seconds earlier, memory demands were increased by gradually lengthening delay intervals (to 120 seconds) and list lengths (to ten objects). With increasing age, only marginal impairments in learning the basic task were observed. However, clear age-related differences did emerge when either delays or list lengths were increased, with the oldest group of monkeys demonstrating the greatest impairments. The decline in visual recognition ability in aging monkeys parallels the decline in memory observed with advancing age in humans.

Senile plaques in aged squirrel monkeys

Aged squirrel monkeys develop senile plaques in the brain that are similar to those occurring in aged rhesus monkeys and aged humans. These plaques consist of abnormal, swollen neurites around an amyloid core. In whole-hemisphere coronal sections through the level of the rostral temporal lobe, plaques are present in temporal cortex, amygdala, hippocampal formation and, occasionally, in other cortical regions. In more rostral sections through the frontal lobe, plaques are most common in orbitofrontal and frontal opercular cortical regions. In immunocytochemical preparations, some neurites show immunoreactivity with antibodies directed against phosphorylated neurofilaments and neuropeptide Y. Thus, plaques in these New World primates are similar in distribution and composition to those occurring in aged Old World primates.

Acetylcholinesterase fibers and the development of senile plaques

A new, highly sensitive histochemical technique for actylcholinesterase (AChE) was applied to a study of Alzheimer's disease brain tissue. Many immature senile plaques were seen to be developing along AChE-positive axons in the hippocampus and neocortex. Single fibers often displayed multiple lesions, showing stages of initial swelling, ballooning with the appearance of AChE in the surrounding extra-axonal space, and development of an AChE-amyloid intense core accompanied by a weakly staining AChE-positive halo. Except for the core and halo, AChE-positive material was seldom detected in so-called mature plaques which are large and incorporate many degenerating neuritic elements surrounding an amyloid core. Lesion data in rats established the relationship between AChE-positive neocortical axons and medial basal forebrain cholinergic cells. In Alzheimer's disease tissue, many degenerating neurons in the basal forebrain were detected by the AChE histochemical stain, along with pathological alterations in the proximal axons en route to their cortical terminal fields. These data provide direct evidence of an association between the cholinergic system of the basal forebrain and the early formation of senile plaques in the cortex in Alzheimer's disease.

The topographic distribution of senile plaques and neurofibrillary tangles in the brains of non-demented persons of different ages

The incidence and severity of senile plaques (SP) and neurofibrillary tangles (NFT) were examined in six areas of brain in 60 non-demented patients of age range 6-84 years. Thirty-two patients showed neither SP nor NFT in any region (30 of these were under 65 years of age), 15 patients showed both SP and NFT in one or more regions (14 were over 60 years of age), 11 patients aged 21-84 years showed NFT only in one or more areas and two patients showed SP alone and only in the amygdala. Overall comparison of number and severity showed the greatest severity of SP within the amygdala in most instances, whereas NFT were found in most instances and at greatest severity within the hippocampus. The reason for this apparent vulnerability of these areas of brain to SP and NFT formation may lie with their connections with the outside world via the olfactory bulbs and tract. This pathway may thus provide an entry point to the brain for pathogenic agent(s) that may induce, either directly or indirectly, pathological processes that ultimately lead to SP and NFT formation.

Conservation of brain amyloid proteins in aged mammals and humans with Alzheimer's disease

The formation of clusters of altered axons and dendrites surrounding extracellular deposits of amyloid filaments (neuritic plaques) is a major feature of the human brain in both aging and Alzheimer's disease. A panel of antibodies against amyloid filaments and their constituent proteins from humans with Alzheimer's disease cross-reacted with neuritic plaque and cerebrovascular amyloid deposits in five other species of aged mammals, including monkey, orangutan, polar bear, and dog. Antibodies to a 28-amino acid peptide representing the partial protein sequence of the human amyloid filaments recognized the cortical and microvascular amyloid of all of the aged mammals examined. Plaque amyloid, plaque neurites, and neuronal cell bodies in the aged animals showed no reaction with antibodies to human paired helical filaments. Thus, with age, the amyloid proteins associated with progressive cortical degeneration in Alzheimer's disease are also deposited in the brains of other mammals. Aged primates can provide biochemically relevant models for principal features of Alzheimer's disease: cerebrovascular amyloidosis and neuritic plaque formation.

An integrative hypothesis concerning the pathogenesis and progression of Alzheimer's disease

Observations, in Alzheimer's disease, in the pattern of nerve cell damage and loss, the pathology, microchemistry and immunology of senile plaques and neurofibrillary tangles and alterations in blood vessels are drawn together into a hypothesis that attempts to explain the pathogenesis and progression of the disorder. At the heart of this hypothesis lies a defect in blood brain barrier function and/or structure within the cerebral cortex and this defect may be the cause of the cerebral vessel amyloidosis common in many patients with Alzheimer's disease. Age-related alterations in blood brain barrier allow for damage to nerve terminals and limited formation of senile plaques within cerebral cortex; neurofibrillary tangles are formed within cortical and subcortical nerve cells which project to or near damaged vessels/senile plaques. Uptake of "neurotoxin" at affected terminals and retrograde transport to perikarya causes neurofibrillary tangles to be formed; their accumulation leads to perikaryal changes culminating in cell death and loss. Loss of cells in cortically projecting areas of subcortex such as nucleus basalis, locus caeruleus and dorsal raphe, which terminate on cerebral vessels, causes further blood brain barrier dysfunction, new plaque formation and continued cell loss in cortex and subcortex. Once started, such a process could be self-perpetuating and the initial site of damage could lie within the amygdala/hippocampus with putative pathogenic agent accessing the brain via the olfactory pathways.