[Differential neuronal loss in the hippocampus in normal aging and in patients with Alzheimer disease]

The causal relationship between the neurodegenerative changes that accompany normal ageing and those that characterize Alzheimer's disease is unclear. The high incidence of Alzheimer's disease associated with old age and the presence of its neuropathological signs in non-demented older individuals suggest that these two phenomena involve the same neurodegenerative processes and mechanisms and that Alzheimer's disease is an extension of normal ageing. On the other hand, the identification of environmental and genetic risk factors associated with Alzheimer's disease suggests the involvement of a specific disease process that is not related to normal ageing. The resolution of this fundamental issue is of importance in the design of investigative and therapeutic strategies. In this report, we describe differences in the regional patterns of neuronal loss, in the hippocampal region of the brains of Alzheimer's patients and normal ageing subjects, that indicate that Alzheimer's disease is not the manifestation of accelerated ageing, but the expression of a distinct pathological process.

Neurons bearing neurofibrillary tangles are responsible for selected synaptic deficits in Alzheimer's disease

The observation that neurons containing neurofibrillary tangles are usually adjacent to neurons free of any morphological indication of disease, suggests the hypothesis that it is NFT-bearing neurons that are primarily responsible for the loss of function in AD. Quantitative Golgi postmortem studies from our laboratories have indicated that there is in many regions of the brains of nondemented humans an age-related increase in dendritic extent of single neurons. In Alzheimer's disease, this normal, age-related increase in dendritic extent was not found, leading to the hypothesis that one of the neurobiological defects in AD is a failure of neuronal plasticity. Message levels of the growth-associated protein, GAP-43, in frontal association cortex (area 9/46) indicated that AD brains with the highest density of neurofibrillary tangle-bearing neurons, showed GAP-43 message levels decreased of the order of 6-fold relative to AD brains with the lowest density of NFT. Combined immunocytochemistry to differentiate tangle-bearing from tangle-free neurons with in situ hybridization to define relative GAP-43 message levels in single neurons revealed that grain density over tangle-bearing neurons containing nuclei was reduced 3-fold compared to that over adjacent tangle-free neurons. This reduction in expression of GAP-43 message in tangle-bearing neurons was selective, because using probes for other messages showed that grain density over tangle-bearing neurons was, on average, increased or similar to that over adjacent non-tangle-bearing neurons. Message levels for the synaptic vesicle-associated protein, synaptophysin, have also been found to be reduced in tangle-bearing neurons relative to adjacent tangle-free neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in the pattern of hippocampal neuronal loss in normal ageing and Alzheimer's disease

The distinction between the neurodegenerative changes that accompany normal ageing and those that characterise Alzheimer's disease is not clear. The resolution of this issue has important implications for the design of therapeutic and investigative strategies. To this end we have used modern stereological techniques to compare the regional pattern of neuronal cell loss in the hippocampus related to normal ageing to that associated with Alzheimer's disease. The loss related to normal ageing was evaluated from estimates of the total number of neurons in each of the major hippocampal subdivisions of 45 normal ageing subjects who ranged in age from 13 to 101 years. The Alzheimer's disease related losses were evaluated from similar data obtained from 7 cases of Alzheimer's disease and 14 age matched controls. Qualitative differences were observed in the regional patterns of neuronal loss related to normal ageing and Alzheimer's disease. The most distinctive Alzheimer's disease related neuron loss was seen in the CA1 region of the hippocampus. In the normal ageing group there was almost no neuron loss in this region (final neuron count in the CA1 region: 4.40 x 10(6) neurons for the Alzheimer's disease group vs 14.08 x 10(6) neurons in the normal ageing group). It is concluded that the neurodegenerative processes associated with normal ageing and with Alzheimer's disease are qualitatively different and that Alzheimer's disease is not accelerated by ageing but is a distinct pathological process.

Preliminary evidence: decreased GAP-43 message in tangle-bearing neurons relative to adjacent tangle-free neurons in Alzheimer's disease parahippocampal gyrus

Loss of synapses has been shown to correlate with the severity of dementia in Alzheimer's disease (AD). Intracellular neurofibrillary tangles (NFTs) have also been shown to correlate to the severity of AD dementia. We have been investigating the influence of NFTs on mRNAs related to neuronal plasticity and synaptic function. We recently reported a decrease in message for the plasticity marker, GAP-43, in AD cases with high tangle densities. The study did not permit us to determine if: a) the decrease in GAP-43 message was specific to the NFT-bearing neurons, b) a general decrease in GAP-43 message was occurring in all surviving neurons, or c) the decrease in GAP-43 message was due to a loss of neurons. It is unlikely a loss of neurons could explain the sixfold GAP-43 message loss we reported, because only a 19% excess decrease in density of hippocampal neurons occurs in AD cases with high tangle densities. Consequently, the study reported here was undertaken to determine if a general decrease in GAP-43 message was occurring in all surviving AD neurons or if the decrease in GAP-43 message was specific to NFT-bearing neurons. We combined immunocytochemistry for neurofibrillary tangles with in situ hybridization for GAP-43 message. We report here preliminary evidence indicating a decrease in GAP-43 message in NFT-bearing neurons compared to adjacent nontangle bearing neurons in parahippocampal cortex of AD patients.

Altered phosphorylation of growth-associated protein B50/GAP-43 in Alzheimer disease with high neurofibrillary tangle density

The growth-associated phosphoprotein B50/GAP-43, associated with axonal proliferation and regeneration, was isolated from superior temporal gyrus (area 22) of seven control and eight Alzheimer disease (AD) postmortem human brains. Membrane and cytoplasmic proteins were fractionated and B50/GAP-43 was isolated by reverse-phase HPLC and gel electrophoresis. B50/GAP-43 was identified with rabbit polyclonal antibodies 4P3 (generated against the calmodulin binding domain of B50/GAP-43) and 1B5 (generated against whole bovine B50/GAP-43). B50/GAP-43 protein was further separated into phosphorylated and dephosphorylated species by calmodulin-Sepharose chromatography. The amounts of phosphorylated and dephosphorylated B50/GAP-43 forms were determined by electrophoresis, protein staining, and densitometry. Data on the relative phosphorylation of B50/GAP-43 protein in membrane and cytoplasmic fractions show a 10-fold difference in the ratio of cytoplasmic/membrane phosphorylation of B50/GAP-43 in AD brains with high neurofibrillary tangle (NFT) density compared to AD brains with low NFT density. This difference is due to a decreased percentage of phosphorylated B50/GAP-43 in the membrane fraction relative to that in the cytosolic fraction from high NFT density. No analogous relationship was found between the phosphorylation of B50/GAP-43 and the density of neuritic plaques in the brains examined. These data indicate differential distribution of phosphorylated and dephosphorylated B50/GAP-43 in normal and AD brains is related to NFT density but not to neuritic plaque density.

Characterization of brain samples in studies of aging, Alzheimer's, and other neurodegenerative diseases

We review current understanding of the clinical and pathologic information needed for the determination of optimal brain tissue samples for the conduct of studies of Alzheimer's disease (AD). Characteristics that may distinguish AD from other dementing disorders are discussed. Selected considerations in the conduct of basic neurobiological studies are also outlined. Although the 28 NIA-funded Alzheimer's Centers can provide excellent clinical and neuropathological data, studies conducted outside these centers should also strive to gather the information suggested here. Clinical and neuropathological data should be used not only to classify subjects as control or AD, but also as variables that may significantly contribute to the analysis of neurobiological data obtained in the laboratory.

Dendritic regression dissociated from neuronal death but associated with partial deafferentation in aging rat supraoptic nucleus

As neurons are lost in normal aging, the dendrites of surviving neighbor neurons may proliferate, regress, or remain unchanged. In the case of age-related dendritic regression, it has been difficult to distinguish whether the regression precedes neuronal death or whether it is a consequence of loss of afferent supply. The rat supraoptic nucleus (SON) represents a model system in which there is no age-related loss of neurons, but in which there is an age-related loss of afferents. The magnocellular neurosecretory neurons of the SON, that produce vasopressin and oxytocin for release in the posterior pituitary, were studied in male Fischer 344 rats at 3, 12, 20, 27, 30, and 32 months of age. Counts in Nissl-stained sections showed no neuronal loss with age, and confirmed similar findings in other strains of rat and in mouse and human. Nucleolar size increased between 3 and 12 months of age, due, in part, to nucleolar fusion, and was unchanged between 12 and 32 months of age, indicating maintenance of general cellular function in old age. Dendritic extent quantified in Golgi-stained tissue increased between 3 and 12 months of age, was stable between 12 and 20 months, and decreased between 20 and 27 months. We interpret the increase between 3 and 12 months as a late maturational change. Dendritic regression between 20 and 27 months was probably the result of deafferentation due to the preceding age-related loss of the noradrenergic input to the SON from the ventral medulla.

The human brain homeogene, DLX-2: cDNA sequence and alignment with the murine homologue

A novel homeobox-containing cDNA from the developing human brain has been cloned and sequenced. The transcript is most closely related to the Distal-less (Dll) homeogene of Drosophila melanogaster and to the Dlx genes in the mouse, specifically to Dlx-2. As such, it is the first report of a human Dll-like gene.

Selective vulnerability and early progression of hippocampal CA1 pyramidal cell degeneration and GFAP-positive astrocyte reactivity in the rat four-vessel occlusion model of transient global ischemia

Selective, delayed-onset vulnerability of hippocampal CA1 pyramidal cells has been reported as a unique phenomenon in man and the rat four-vessel occlusion (4-VO) model of global ischemia. This has become of great interest for clarification of CA1 pathophysiology and pharmacological intervention after global ischemia. Studies of pathophysiology and pharmacotherapy appear to be impeded by variability in specific criteria and duration of 4-VO ischemia for producing selective CA1 and differential CA1-CA3 damage. The goals of this study were to: (1) develop specific criteria for 4-VO ischemia to ensure selective, bilaterally symmetrical CA1 pyramidal cell damage, (2) examine the effects of 15 min of ischemia on concomitant CA1 cell necrosis and presence of remaining and/or "viable" neurons postischemia, (3) compare 15 and 30 min of ischemia on differential vulnerability of CA1-CA3 subfields, and (4) evaluate the effects of 15 min of ischemia on CA1 pyramidal cell necrosis and glial fibrillary acidic protein (GFAP)-positive astrocyte reactivity in CA1. After 15 min of ischemia, hippocampal pyramidal cell damage was well delineated, with CA1 severely damaged, but leaving CA3 virtually intact. In contrast, 30 min of ischemia produced severe CA1 and less severe CA3 necrosis. Histological evaluations across Days 1, 3, 6, and 14 indicated a significant delayed onset of CA1-CA3 cell necrosis by Day 3. Counting of remaining cells indicated a detectable loss of some large pyramidal neurons even 1 day after ischemia. Compared to controls, there was a differential increase in GFAP-positive astrocytes in CA1-CA3 after ischemia. The results provided quantitative data on the effects of specific 4-VO criteria and durations on: (1) selective CA1 cell necrosis, (2) differential CA1-CA3 cell vulnerability, (3) presence of postischemic remaining and/or viable neurons, and (4) prospect of a "therapeutic window" for pharmacological treatment of CA1 neuronal injury.

Reduced GAP-43 message levels are associated with increased neurofibrillary tangle density in the frontal association cortex (area 9) in Alzheimer's disease

We previously suggested the hypothesis that defective neuronal plasticity is a major neurobiological deficit causing the dementia of Alzheimer's disease (AD). We used message levels of the growth-associated protein, GAP-43, as a marker of axonal plasticity to examine the hypothesis of defective neuronal plasticity in AD. When all AD cases are combined, the average level of GAP-43 message in area 9 of the AD frontal association cortex was not significantly different from the level in the comparably aged control cortex. Differentiation of AD cases on the basis of neurofibrillary tangle (NFT) density revealed that in AD cases with high tangle density average GAP-43 message level was reduced fivefold relative to levels in AD cases with low NFT density. AD cases with low neurofibrillary tangle density had levels of GAP-43 message that were not significantly different from the levels of normal controls. Differentiation of AD cases on the basis of neuritic plaque density did not indicate as strong a relationship to GAP-43 message level. The association between neurofibrillary tangle density and GAP-43 message level suggests the hypothesis that neurofibrillary tangles may reduce GAP-43 expression. Data of others show a relationship between high NFT density and reduced levels of synaptophysin-like immunoreactivity and reduced cerebral glucose metabolism. These data combine to suggest a set of AD cases with high NFT density, reduced axonal plasticity, reduced synaptic density, and reduced cerebral glucose metabolism--all variables that may be directly related to the functioning of the brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Divergence of biological and chronological aging: evidence from rodent studies

Literature on aging populations of rodents supports the intuitive view that significant functional variation exists among like-aged, elderly individuals: chronological age as a solitary measure is a poor indicator of biological age. In this report, we review a variety of studies which classify aged rodents based on genetic and/or behavioral similarities, in addition to chronological age, and have provided valuable neurobiological and physiological information on age-related changes which accompany functional impairments, or the lack of them. Beyond their descriptive value for gerontological research, these findings suggest ways in which biological aging can be manipulated to promote good function in aged individuals.

Net dendritic stability of layer II pyramidal neurons in F344 rat entorhinal cortex from 12 to 37 months

Dendritic extent of Golgi-Cox stained layer II entorhinal cortex pyramidal neurons was quantified in five groups of male F344 rats aged 12, 20, 27, 30 and 37 months. Over the age range studied, neither the apical nor the basal dendritic trees showed any statistically significant change in total dendritic length, numbers of segments or average segment length. This finding of average stability of the dendritic tree does not imply absence of remodelling of connections, but does require that if remodeling does occur, retraction and proliferation of dendrites must, on average, be equal. We hypothesized that in groups of animals with similar genetic and environmental histories neighbor neuron death provides the major stimulus for dendritic proliferation. Since we found dendritic stability in the cells reported here, we would predict that there should be no age-related loss of layer II pyramidal neurons in the entorhinal cortex of the normally aging F344 male rat between 12 and 37 months. This hypothesis may be tested by counting neurons within this region.

Hippocampal plasticity in normal aging and decreased plasticity in Alzheimer's disease

Different patterns of age-related dendritic change have been reported in different zones of the human hippocampal region in the normal and Alzheimer's disease (AD) brain. In normal aging there is an increase in average (net) dendritic extent (which we interpret as plasticity) in the parahippocampal gyrus and dentate gyrus. There is net stability of dendritic extent in CA2-3, CA1, and subiculum. In regions that show plasticity in normal aging, dendrites in AD show reduced or aberrant plasticity. In regions that show stability in normal aging, dendrites either are stable or regress in AD, depending upon how severely involved the region is with the pathology of AD.

Neuron numbers and sizes in aging brain: comparisons of human, monkey, and rodent data

One of the several sources of interest in aging animal brains is their potential as models of the aging human brain. In this review we examine whether neuron numbers and sizes change similarly in aging human, monkey and rodent brain regions which data are available from more than one species. The number of brain regions studied in more than one species is surprisingly limited. Some regions show correspondence in age-related changes between humans and selected animal models (primary visual cortex, CA1 of hippocampus). For the majority of regions the data are conflicting, even within one species (e.g., somatosensory cortex, frontal cortex, cerebellum, cholinergic forebrain areas, locus coeruleus). Although some of the conflicting data may be attributed to procedural differences, particularly when data are expressed as density changes, much must be attributed to real species and/or strain differences in rodents. We conclude that neuron numbers and sizes may show similar age-related changes in human and animal brains only for sharply defined brain regions, animal species and/or strains, and age ranges.

Estimating the number of granule cells in the dentate gyrus with the disector

A practical example is given of how a newly developed stereological estimator of particle number, the disector, can be used to make estimates of neuron number in the dentate gyrus of rats. The estimates are free of biases related to lost caps, overprojection and assumptions about size, shape and orientation of the objects that are counted. The disector principle and the practical considerations relating to histological preparations and sampling are presented.

Volumes of the components of the hippocampus in the aging F344 rat

Much of the recent data on cells, synapses, and other structures in the dentate gyrus and hippocampus as a function of age are packing density or volume fraction data. In order to estimate total numbers, volumes, or surface areas of cells, synapses, vessels, etc., as a function of age, the total volumes of the subregions of the dentate gyrus and hippocampus must be known. The volumes of these subregions, visualized with the Timm stain, have been determined in 24 F344 rats from 4 to 37 months of age. Volumes of the various structures showed age-related increases which were statistically significant for the perforant path zone of the dentate gyrus molecular layer, as well as the total molecular layer, the hilus, and regio inferior and total mossy fiber systems. If the 4-month age group is eliminated from consideration, only the ratio of the volume of the mossy fiber zones to the volume of the perforant path zones of the dentate molecular layer increases significantly with age. Our general finding of lack of volumetric reorganization of the subdivisions of the hippocampal region between 12 and 37 months suggests that studies of the packing densities of structures in most of these zones may be considered comparable across ages, assuming comparability of sampling regions.

Neuron numbers and dendritic extent in normal aging and Alzheimer's disease

Factors which limit the interpretation of studies of aging brain include: secular trends, species and strain differences, effects of tissue processing, and bias which may be introduced at many levels of an experimental design. With these limitations considered, evidence is reviewed regarding neuron numbers and dendritic extent in normally aging rodent, monkey and human brain and in Alzheimer's disease. It is concluded that neuron loss and change in dendritic extent in normal aging are regionally specific, and that corresponding brain regions do not always change in similar ways in rodents and primates. It is suggested that such differences may, in part, be due to inconsistent definitions of 'aged' among species. In Alzheimer's disease there is excess neuron loss and dendritic regression in some, but not all, brain regions. Measures of the morphological substrates of brain function show appreciable overlap between AD and control groups. It is hypothesized that the static, post-mortem status of brain morphology may not adequately reflect the functional capabilities of the dynamic morphology of the living brain.

Dendritic extent in human CA2-3 hippocampal pyramidal neurons in normal aging and senile dementia

The extent of dendritic trees of pyramidal neurons of the CA2-3 field of the hippocampus of 20 human brains obtained at autopsy was quantified in Golgi Cox-stained tissue. Fifteen cases were neurologically and psychiatrically normal and ranged in age from 43 to 95 years. Five cases had a progressive, dementing disease consistent with the diagnosis of senile dementia (SD) of the Alzheimer's type. Dendritic extent of both the apical and basal trees of CA2-3 pyramidal neurons was found to be unchanged from middle age to very old age. This finding of net stability of dendritic extent is in contrast to previous quantitative reports of either continued dendritic growth in human parahippocampal gyrus or of dendritic growth followed by regression in human dentate gyrus. This finding is consistent with the suggestion that changes in dendritic extent in normal aging are a function of the balance between regressive and proliferative influences and are region specific. In cases with SD, dendritic extent of both the apical and basal trees was found to be similar to that of the normal age-matched cases. These data are consistent with those of others suggesting relative sparing of the CA2-3 field from the degenerative changes in senile dementia.