Age-related gliosis in the white matter of mice

Dietary energy restriction in neurological diseases: what's new?

Energy-restricted diet is a specific dietary regimen, including the continuous energy-restricted diet and the intermittent energy-restricted diet. It has been proven effective not only to reduce weight and extend the lifespan in animal models, but also to regulate the development and progression of various neurological diseases such as epilepsy, cerebrovascular diseases (stroke), neurodegenerative disorders (Alzheimer's disease and Parkinson's disease) and autoimmune diseases (multiple sclerosis). However, the mechanism in this field is still not clear and a systematic neurological summary is still missing. In this review, we first give a brief summary of the definition and mainstream strategies of energy restrictions. We then review evidence about the effects of energy-restricted diet from both animal models and human trials, and update the current understanding of mechanisms underlying the biological role of energy-restricted diet in the fight against neurological diseases. Our review thus contributes to the modification of dietary regimen and the search for special diet mimics.

Brain and spinal cord lesions in 28 inbred strains of aging mice

Brain and spinal cord histopathology findings in male and female 20-month-old mice in a large-scale aging study of 28 inbred Jackson Laboratory mouse strains from 7 genetic families are described. Brain sections from selected strains at 12 and 24 months of age or older were also reviewed. Common lesions include axonal dystrophy in the gracile and/or cuneate nucleus in the sensory tract of the dorsal medulla and in the spinal cord in all strains. Hirano-like bodies were seen in 24/28 strains, and mineralization was observed in the thalamus of 9/28 strains. Less common lesions were also seen in the cerebellum, cerebral cortex, and other brain areas. No brain or spinal cord tumors were found. Evidence of an impairment of the ubiquitin-proteasome system (UPS) and/or suspected autophagy was manifested as medullary axonal dystrophy with intra-axonal granular eosinophilic bodies and LC3B immunohistochemistry in most strains. RIIIS/J, the most severely affected strain, showed moderate axonal dystrophy at 12 months, which progressed to severe lesions at 20 months. Comparative pathology in various species is discussed.

Principles of brain aging: Status and challenges of modeling human molecular changes in mice

Due to the extension of human life expectancy, the prevalence of cognitive impairment is rising in the older portion of society. Developing new strategies to delay or attenuate cognitive decline is vital. For this purpose, it is imperative to understand the cellular and molecular events at the basis of brain aging. While several organs are directly accessible to molecular analysis through biopsies, the brain constitutes a notable exception. Most of the molecular studies are performed on postmortem tissues, where cell death and tissue damage have already occurred. Hence, the study of the molecular aspects of cognitive decline largely relies on animal models and in particular on small mammals such as mice. What have we learned from these models? Do these animals recapitulate the changes observed in humans? What should we expect from future mouse studies? In this review we answer these questions by summarizing the state of the research that has addressed cognitive decline in mice from several perspectives, including genetic manipulation and omics strategies. We conclude that, while extremely valuable, mouse models have limitations that can be addressed by the optimal design of future studies and by ensuring that results are cross-validated in the human context.

Spontaneous Incidental Brain Lesions in C57BL/6J Mice

Genetically engineered mouse lines on a C57BL/6J background are widely employed as preclinical models to study neurodegenerative human disorders and brain tumors. However, because of the lack of comprehensive data on the spontaneous background neuropathology of the C57BL/6J strain, discriminating between naturally occurring changes and lesions caused by experimental mutations can be challenging. In this context, this study aims at defining the spectrum and frequency of spontaneous brain changes in a large cohort of C57BL/6J mice and their association with specific biological variables, including age and sex. Brains from 203 experimentally naive and clinically unremarkable C57BL/6J mice were collected and analyzed by means of histopathology and immunohistochemistry. Mice ranged in age from 3 to 110 weeks with 89 females, 111 males, and 3 unknowns. Sixteen different spontaneous lesion categories were described in this cohort. Age-related neurodegenerative and/or neuroinflammatory findings represented the most common pathologic changes and included (1) Hirano-like inclusions in the thalamic neurons, (2) neuroaxonal dystrophy in the medulla oblongata, (3) periodic acid-Schiff-positive granular deposits in the neuropil of the hippocampus, and (4) progressive neuroinflammation characterized by microgliosis and astrogliosis. Neoplastic conditions, developmental abnormalities, and circulatory disorders were rarely observed incidental findings. In conclusion, this study describes spontaneous age-related brain lesions of the C57BL/6J mouse and provides a reference for evaluating and interpreting the neuropathological phenotype in genetically engineered mouse models developed and maintained on this congenic background.

FOXO protects against age-progressive axonal degeneration

Neurodegeneration resulting in cognitive and motor impairment is an inevitable consequence of aging. Little is known about the genetic regulation of this process despite its overriding importance in normal aging. Here, we identify the Forkhead Box O (FOXO) transcription factor 1, 3, and 4 isoforms as a guardian of neuronal integrity by inhibiting age-progressive axonal degeneration in mammals. FOXO expression progressively increased in aging human and mouse brains. The nervous system-specific deletion of Foxo transcription factors in mice accelerates aging-related axonal tract degeneration, which is followed by motor dysfunction. This accelerated neurodegeneration is accompanied by levels of white matter astrogliosis and microgliosis in middle-aged Foxo knockout mice that are typically only observed in very old wild-type mice and other aged mammals, including humans. Mechanistically, axonal degeneration in nerve-specific Foxo knockout mice is associated with elevated mTORC1 activity and accompanying proteotoxic stress due to decreased Sestrin3 expression. Inhibition of mTORC1 by rapamycin treatment mimics FOXO action and prevented axonal degeneration in Foxo knockout mice with accelerated nervous system aging. Defining this central role for FOXO in neuroprotection during mammalian aging offers an invaluable window into the aging process itself.

Aging affects new cell production in the adult hippocampus: A quantitative anatomic review

In the last century, cognitive impairment in elderly people was considered as the consequence of neuronal death. However, later analyses indicated that age-related reduction in neuron number was limited to specific regions of the central nervous system, and was irrelevant to brain dysfunction in both humans and non-human animals. Recent studies have indicated that progressive diminution of neural plasticity across an individual's life span may underlie age-related brain dysfunction. To date, various factors have been shown to contribute to neural plasticity. In particular, substantial data supports the importance of production of new cells in the adult brain: the rate of hippocampal neurogenesis wanes radically during aging; similarly, white matter homeostasis via oligodendrogenesis is also affected by aging. This review briefly summarizes quantitative studies on adult hippocampal neurogenesis and oligodendrogenesis. Although the hippocampus is traditionally recognized as the memory center of the brain, it has started to emerge as an integrator of cognition and emotion. One of the current research highlights is that diverse functions of the hippocampus are topographically embedded along its longitudinal and transverse axes. Here we discuss alterations in adult neurogenesis and oligodendrogenesis during aging from a topographic view point. The quantitative anatomic approach to age-related alterations in production of new cells in the hippocampus may give a novel insight into how brain functions suffer from aging.

Loss of Nfkb1 leads to early onset aging

NF-κB is a major regulator of age-dependent gene expression and the p50/NF-κB1 subunit is an integral modulator of NF-κB signaling. Here, we examined Nfkb1-/- mice to investigate the relationship between this subunit and aging. Although Nfkb1-/- mice appear similar to littermates at six months of age, by 12 months they have a higher incidence of several observable age-related phenotypes. In addition, aged Nfkb1-/- animals have increased kyphosis, decreased cortical bone, increased brain GFAP staining and a decrease in overall lifespan compared to Nfkb1+/+. In vitro, serially passaged primary Nfkb1-/- MEFs have more senescent cells than comparable Nfkb1+/+ MEFs. Also, Nfkb1-/- MEFs have greater amounts of phospho-H2AX foci and lower levels of spontaneous apoptosis than Nfkb1+/+, findings that are mirrored in the brains of Nfkb1-/- animals compared to Nfkb1+/+. Finally, in wildtype animals a substantial decrease in p50 DNA binding is seen in aged tissue compared to young. Together, these data show that loss of Nfkb1 leads to early animal aging that is associated with reduced apoptosis and increased cellular senescence. Moreover, loss of p50 DNA binding is a prominent feature of aged mice relative to young. These findings support the strong link between the NF-κB pathway and mammalian aging.

A stereological analysis of NPY, POMC, Orexin, GFAP astrocyte, and Iba1 microglia cell number and volume in diet-induced obese male mice

The hypothalamic arcuate nucleus (ARC) contains 2 key neural populations, neuropeptide Y (NPY) and proopiomelanocortin (POMC), and, together with orexin neurons in the lateral hypothalamus, plays an integral role in energy homeostasis. However, no studies have examined total neuronal number and volume after high-fat diet (HFD) exposure using sophisticated stereology. We used design-based stereology to estimate NPY and POMC neuronal number and volume, as well as glial fibrillary acidic protein (astrocyte marker) and ionized calcium-binding adapter molecule 1 (microglia marker) cell number in the ARC; as well as orexin neurons in the lateral hypothalamus. Stereological analysis indicated approximately 8000 NPY and approximately 9000 POMC neurons in the ARC, and approximately 7500 orexin neurons in the lateral hypothalamus. HFD exposure did not affect total neuronal number in any population. However, HFD significantly increased average NPY cell volume and affected NPY and POMC cell volume distribution. HFD reduced orexin cell volume but had a bimodal effect on volume distribution with increased cells at relatively small volumes and decreased cells with relatively large volumes. ARC glial fibrillary acidic protein cells increased after 2 months on a HFD, although no significant difference after 6 months on chow diet or HFD was observed. No differences in ARC ionized calcium-binding adapter molecule 1 cell number were observed in any group. Thus, HFD affects ARC NPY or POMC neuronal cell volume number not cell number. Our results demonstrate the importance of stereology to perform robust unbiased analysis of cell number and volume. These data should be an empirical baseline reference to which future studies are compared.

Impact of age-related neuroglial cell responses on hippocampal deterioration

Aging is one of the greatest risk factors for the development of sporadic age-related neurodegenerative diseases and neuroinflammation is a common feature of this disease phenotype. In the immunoprivileged brain, neuroglial cells, which mediate neuroinflammatory responses, are influenced by the physiological factors in the microenvironment of the central nervous system (CNS). These physiological factors include but are not limited to cell-to-cell communication involving cell adhesion molecules, neuronal electrical activity and neurotransmitter and neuromodulator action. However, despite this dynamic control of neuroglial activity, in the healthy aged brain there is an alteration in the underlying neuroinflammatory response notably seen in the hippocampus, typified by astrocyte/microglia activation and increased pro-inflammatory cytokine production and signaling. These changes may occur without any overt concurrent pathology, however, they typically correlate with deteriorations in hippocamapal or cognitive function. In this review we examine two important phenomenons, firstly the relationship between age-related brain deterioration (focusing on hippocampal function) and underlying neuroglial response(s), and secondly how the latter affects molecular and cellular processes within the hippocampus that makes it vulnerable to age-related cognitive decline.

Indeterminate body growth and lack of gonadal decline in the brown ghost knifefish (Apteronotus leptorhynchus), an organism exhibiting negligible brain senescence

The brown ghost knifefish (Apteronotus leptorhynchus (Ellis in Eigenmann, 1912)) is the only vertebrate organism identified thus far that exhibits negligible brain senescence. The present study examines the basic growth patterns of this species, testing the hypothesis that indeterminate growth and lack of reproductive senescence correlate with negligible senescence. Analysis of length–mass relationships revealed negative allometric growth in males and isometric growth in females. Total length at first sexual maturity was 13.5 cm in males and 12.0 cm in females, whereas gonadal mass was 0.02 g in males and 0.2 g in females. Modelling of total length as a function of the number of otolith rings using attenuating growth equations revealed that lengths of up to 26.8 cm in males and 20.2 cm in females can be reached, indicating that the fish continue to grow throughout life. Gonadal mass increased significantly with age in sexually immature individuals of both sexes. In sexually mature fish, gonadal mass showed a marginal increase with age in males and no change in females. The demonstration of indeterminate growth of the fish and of the lack of gonadal regression with age has important implications for the characterization of brown ghost knifefish as a model of negligible senescence.

Age-related differences in oligodendrogenesis across the dorsal-ventral axis of the mouse hippocampus

Oligodendrocyte precursor cells (OPCs) continue to divide and generate new oligodendrocytes (OLs) in the healthy adult brain. Although recent studies have indicated that adult oligodendrogenesis may be vital for the maintenance of normal brain function, the significance of adult oligodendrogenesis in brain aging remains unclear. In this study, we report a stereological estimation of age-related oligodendrogenesis changes in the mouse hippocampus: the dorsal subdivision is related to learning and memory, while the ventral subdivision is involved in emotional behaviors. To identify OPCs and OLs, we used a set of molecular markers, OL lineage transcription factor (Olig2) and platelet-derived growth factor receptor-alpha (PDGFαR). Intracellular dye injection shows that PDGFαR+/Olig2+ cells and PDGFαR-/Olig2+ cells can be defined as OPCs and OLs, respectively. In the dorsal Ammon's horn, the numbers of OPCs decreased with age, while those of OLs remained unchanged during aging. In the ventral Ammon's horn, the numbers of OPCs and OLs generally decreased with age. Bromodeoxyuridine (BrdU) fate-tracing analysis revealed that the numbers of BrdU+ mitotic OPCs in the Ammon's horn remained unchanged during aging in both the dorsal and ventral subdivisions. Unexpectedly, the numbers of BrdU+ newly generated OLs increased with age in the dorsal Ammon's horn, but remained unchanged in the ventral Ammon's horn. Together, the numbers of OLs in the dorsal Ammon's horn may be maintained during aging by increased survival of adult born OLs, while the numbers of OLs in the ventral Ammon's horn may be reduced with age due to the lack of such compensatory mechanisms. These observations provide new insight into the involvement of adult oligodendrogenesis in age-related changes in the structure and function of the hippocampus.

Gray matter alterations in early aging: a diffusion magnetic resonance imaging study

Many studies have observed altered neurofunctional and structural organization in the aging brain. These observations from functional neuroimaging studies show a shift in brain activity from the posterior to the anterior regions with aging (PASA model), as well as a decrease in cortical thickness, which is more pronounced in the frontal lobe followed by the parietal, occipital, and temporal lobes (retrogenesis model). However, very little work has been done using diffusion MRI (dMRI) with respect to examining the structural tissue alterations underlying these neurofunctional changes in the gray matter. Thus, for the first time, we propose to examine gray matter changes using diffusion MRI in the context of aging. In this work, we propose a novel dMRI based measure of gray matter "heterogeneity" that elucidates these functional and structural models (PASA and retrogenesis) of aging from the viewpoint of diffusion MRI. In a cohort of 85 subjects (all males, ages 15-55 years), we show very high correlation between age and "heterogeneity" (a measure of structural layout of tissue in a region-of-interest) in specific brain regions. We examine gray matter alterations by grouping brain regions into anatomical lobes as well as functional zones. Our findings from dMRI data connects the functional and structural domains and confirms the "retrogenesis" hypothesis of gray matter alterations while lending support to the neurofunctional PASA model of aging in addition to showing the preservation of paralimbic areas during healthy aging.

Age-related changes in stem cell dynamics, neurogenesis, apoptosis, and gliosis in the adult brain: a novel teleost fish model of negligible senescence

Adult neurogenesis, the generation of new neurons in the adult central nervous system, is a reported feature of all examined vertebrate species. However, a dramatic decline in the rates of cell proliferation and neuronal differentiation occurs in mammals, typically starting near the onset of sexual maturation. In the present study, we examined possible age-related changes associated with adult neurogenesis in the brain of brown ghost knifefish (Apteronotus leptorhynchus), a teleost fish distinguished by its enormous neurogenic potential. Contrary to the well-established alterations in the mammalian brain during aging, in the brain of this teleostean species we could not find evidence for any significant age-related decline in the absolute levels of stem/progenitor cell proliferation, neuronal and glial differentiation, or long-term survival of newly generated cells. Moreover, there was no indication that the amount of glial fibrillary acidic protein or the number of apoptotic cells in the brain was altered significantly over the course of adult life. We hypothesize that this first demonstration of negligible cellular senescence in the vertebrate brain is related to the continued growth of this species and to the lack of reproductive senescence during adulthood. The establishment of the adult brain of this species as a novel model of negligible senescence provides new opportunities for the advancement of our understanding of the biology of aging and the fundamental mechanisms that underlie senescence in the brain.

Dementia in Parkinson's Disease Correlates with α-Synuclein Pathology but Not with Cortical Astrogliosis

Dementia is a common feature in Parkinson's disease (PD) and is considered to be the result of limbic and cortical Lewy bodies and/or Alzheimer changes. Astrogliosis may also affect the development of dementia, since it correlates well with declining cognition in Alzheimer patients. Thus, we determined whether cortical astrogliosis occurs in PD, whether it is related to dementia, and whether this is reflected by the presence of glial fibrillary acidic protein (GFAP) and vimentin in cerebrospinal fluid (CSF). We have examined these proteins by immunohistochemistry in the frontal cortex and by Western blot in CSF of cases with PD, PD with dementia (PDD), dementia with Lewy bodies (DLB) and nondemented controls. We were neither able to detect an increase in cortical astrogliosis in PD, PDD, or DLB nor could we observe a correlation between the extent of astrogliosis and the degree of dementia. The levels of GFAP and vimentin in CSF did not correlate to the extent of astrogliosis or dementia. We did confirm the previously identified positive correlation between the presence of cortical Lewy bodies and dementia in PD. In conclusion, we have shown that cortical astrogliosis is not associated with the cognitive decline in Lewy body-related dementia.

Regional and laminar differences in antigen profiles and spatial distributions of astrocytes in the mouse hippocampus, with reference to aging

The heterogeneity of astrocytes is of growing interest, because this information is now considered to be crucial for understanding the diverse roles of astrocytes, for example, support and nutrition for neurons, and modulation of synaptic plasticity. In this study, we stereologically estimated the regional and laminar differences in antigen profiles and spatial distributions of astrocytes in the young adult (2-month-old) and middle-aged (10-month-old) mouse hippocampus. Here we used two established astrocyte markers, that is, glial fibrillary acidic protein (GFAP) and S100β, to identify the astrocyte population. In addition, we examined the patterns of expression of sex determining region Y-box 2 (Sox2) in the hippocampus. The majority of astrocytes expressed Sox2, and few regional and laminar differences were observed in the expression ratios of Sox2 in astrocytes. GFAP-negative astrocytes were specifically seen in the strata pyramidale and lucidum of the ventral CA3 region. S100β-negative astrocytes were mainly found in the hilus of the dorsal and ventral dentate gyrus. Antigen profiles of astrocytes defined by Sox2, GFAP, and S100β were rather constant until middle age. We then estimated the heterogeneity in spatial distributions of astrocytes. The numbers of astrocytes in the stratum lacunosum-molecular of the dorsal part of Ammon's horn were significantly larger in the middle-aged mice than in young adult mice. On the contrary, the astrocyte numbers in the stratum oriens of Ammon's horn showed significant age-dependent decline. Despite such changes, the total number of astrocytes in the whole area of the hippocampus showed no differences between young adult and middle-aged mice. The present data may work as an essential anatomical reference to understand the heterogeneity of astrocytes in the hippocampus.

Effects of dietary restriction on mortality and age-related phenotypes in the short-lived fish Nothobranchius furzeri

The short-lived annual fish Nothobranchius furzeri shows extremely short captive life span and accelerated expression of age markers, making it an interesting model system to investigate the effects of experimental manipulations on longevity and age-related pathologies. Here, we tested the effects of dietary restriction (DR) on mortality and age-related markers in N. furzeri. DR was induced by every other day feeding and the treatment was performed both in an inbred laboratory line and a longer-lived wild-derived line. In the inbred laboratory line, DR reduced age-related risk and prolonged maximum life span. In the wild-derived line, DR induced early mortality, did not reduce general age-related risk and caused a small but significant extension of maximum life span. Analysis of age-dependent mortality revealed that DR reduced demographic rate of aging, but increased baseline mortality in the wild-derived strain. In both inbred- and wild-derived lines, DR prevented the expression of the age markers lipofuscin in the liver and Fluoro-Jade B (neurodegeneration) in the brain. DR also improved performance in a learning test based on conditioning (active avoidance in a shuttle box). Finally, DR induced a paradoxical up-regulation of glial fibrillary acidic protein in the brain.

Age-related changes in glial cells of dopamine midbrain subregions in rhesus monkeys

Aging remains the strongest risk factor for developing Parkinson's disease (PD), and there is selective vulnerability in midbrain dopamine (DA) neuron degeneration in PD. By tracking normal aging-related changes with an emphasis on regional specificity, factors involved in selective vulnerability and resistance to degeneration can be studied. Towards this end, we sought to determine whether age-related changes in microglia and astrocytes in rhesus monkeys are region-specific, suggestive of involvement in regional differences in vulnerability to degeneration that may be relevant to PD pathogenesis. Gliosis in midbrain DA subregions was measured by estimating glia number using unbiased stereology, assessing fluorescence intensity for proteins upregulated during activation, and rating morphology. With normal aging, microglia exhibited increased staining intensity and a shift to more activated morphologies preferentially in the vulnerable substantia nigra-ventral tier (vtSN). Astrocytes did not exhibit age-related changes consistent with an involvement in regional vulnerability in any measure. Our results suggest advancing age is associated with chronic mild inflammation in the vtSN, which may render these DA neurons more vulnerable to degeneration.

Delayed neurodegeneration and early astrogliosis after excitotoxicity to the aged brain

Excitotoxicity is well recognised as a mechanism underlying neuronal cell death in several brain injuries. To investigate age-dependent differences in neurodegeneration, edema formation and astrogliosis, intrastriatal N-methyl-d-aspartate injections were performed in young (3 months) and aged (22-24 months) male Wistar rats. Animals were sacrificed at different times between 12h and 14 days post-lesion (DPL) and cryostat sections were processed for Toluidine blue, Fluoro-Jade B staining, NeuN and GFAP immunohistochemistry. Our results show that both size of tissue injury and edema were reduced in the old subjects only up to 1DPL, correlating with a slower progression of neurodegeneration with peak numbers of degenerating neurons at 3DPL in the aged, contrasting with maximum neurodegeneration at 1DPL in the young. However, old animals showed an earlier onset of astroglial response, seen at 1DPL, and a larger area of astrogliosis at all time-points studied, including a greater glial scar. In conclusion, after excitotoxic striatal damage, progression of neurodegeneration is delayed in the aged but the astroglial response is earlier and exacerbated. Our results emphasize the importance of using aged animals and several survival times for the study of acute age-related brain insults.

Mutant mice with small amounts of BubR1 display accelerated age-related gliosis

Aging is an intricate biological process thought to involve multiple molecular pathways. The spindle assembly checkpoint protein BubR1 has recently been implicated in aging since mutant mice that have small amounts of this protein (BubR1(H/H) mice) develop several early aging-associated phenotypes. The phenotype within the brain of BubR1(H/H) mice has not yet been established. Here we show that BubR1(H/H) mice exhibit features of age-related cerebral degeneration. We found that glial fibrillary acidic protein (GFAP), a marker of reactive astrogliosis, was expressed at increased levels in the cortex and thalamus of BubR1(H/H) mice as early as 1 month of age. Furthermore, CD11b, a marker of microgliosis, was markedly elevated in the cortex and hippocampus of BubR1(H/H) mice at 5 months of age. Levels of both GFAP and CD11b further increased with age. Our results demonstrate that BubR1 acts to prevent cerebral gliosis of both astrocytes and microglial cells, and suggest a role for BubR1 in the aging process of the brain.

Regional neuropathology following kainic acid intoxication in adult and aged C57BL/6J mice

We evaluated regional neuropathological changes in adult and aged male mice treated systemically with kainic acid (KA) in a strain reported to be resistant to excitotoxic neuronal damage, C57BL/6. KA was administered in a single intraperitoneal injection. Adult animals were dosed with 35 mg/kg KA, while aged animals received a dose of 20 mg/kg in order to prevent excessive mortality. At time-points ranging from 12 h to 7 days post-treatment, animals were sacrificed and prepared for histological evaluation utilizing the cupric-silver neurodegeneration stain, immunohistochemistry for GFAP and IgG, and lectin staining. In animals of both ages, KA produced argyrophilia in neurons throughout cortex, hippocampus, thalamus, and amygdala. Semi-quantitative analysis of neuropathology revealed a similar magnitude of damage in animals of both ages, even though aged animals received less toxicant. Additional animals were evaluated for KA-induced reactive gliosis, assayed by an ELISA for GFAP, which revealed a 2-fold elevation in protein levels in adult mice, and a 2.5-fold elevation in aged animals. Histochemical evaluation of GFAP and lectin staining revealed activation of astrocytes and microglia in regions with corresponding argyrophilia. IgG immunostaining revealed a KA-induced breach of the blood-brain barrier in animals of both ages. Our data indicate widespread neurotoxicity following kainic acid treatment in C57BL/6J mice, and reveal increased sensitivity to this excitotoxicant in aged animals.

The ageing phenome: caloric restriction and hormones promote neural cell survival, growth, and de-differentiation

The phenome represents the observable properties of an organism that have developed under the continued influences of both genome and environmental factors. Phenotypic properties are expressed through the functions of cells, organs and body systems that operate optimally, close to equilibrium. In complex organisms, maintenance of the equilibrium is achieved by the interplay of several regulatory mechanisms. In the elderly, dynamic instability may lead to progressive loss of normal function, failure of adaptation and increased pathology. Extensive research (reported elsewhere in this journal) has demonstrated that genetic manipulations of endocrine signaling in flies, worms and mice increase longevity. Another effective strategy for prolonging the lifespan is caloric restriction: in data presented here, the persistence of estrogen-sensitive cells in the hypothalamus of caloric restricted 22-month-old female mice, may explain the persistence of reproductive function at an age, when reproductive function has long ceased in ad libitum fed controls. Still another strategy utilizes the effects of epidermal growth factor (EGF) to promote in vitro proliferation of neuroglia, astrocytes and oligodendrocytes. Their subsequent de-differentiation generates immature precursor cells potentially capable of differentiating into neuroblasts and neurons. These and other examples suggest that, in terms of functional outcomes, "the genome proposes but the phenome disposes".

Effect of aging on the distribution of basic fibroblast growth factor immunoreactive cells in the rat hippocampus

Hippocampal formation is extremely sensitive to the aging process and appears to be one of the first regions to show structural and physiological changes with advancing age. Basic fibroblast growth factor (bFGF) plays an important role in the stimulation of mitogenesis in glial cells, the support of neuronal survival and the promotion of neurite outgrowth in vitro. In the present study, the effect of aging on the distribution of bFGF immunoreactive (bFGF-ir) cells was investigated. The protein product of bFGF was visualized immunohistochemically in the dorsal hippocampus of Wistar albino rats. bFGF-ir astrocytes in different subfields of hippocampus and neurons in CA2 field were quantified to determine whether changes in immunoreactivity were correlated with advancing age. Aging was accompanied by a decrease in bFGF-ir cell density in subfields of hippocampus. We concluded that aging was associated with a reduction in bFGF-ir cell density that may reflect a decreased expression of bFGF in the rat hippocampus.

Glial fibrillary acidic protein mRNA levels in the cingulate cortex of individuals with depression, bipolar disorder and schizophrenia

Recent studies have shown a decrease in glial number and glial fibrillary acidic protein (GFAP) levels in the frontal and cingulate cortices of individuals with mood disorders and schizophrenia. In an attempt to verify and expand these findings we examined GFAP messenger ribonucleic acid (mRNA) levels in postmortem sections of the anterior cingulate cortex (ACC) from the Stanley Neuropathology Consortium (SNC). The consortium consists of 15 cases in each of four groups (schizophrenia, bipolar disorder, non-psychotic depression and unaffected controls). By in situ hybridization, we found higher levels of GFAP mRNA in white matter and at the pial surface as compared with gray matter levels in all cases. In the white matter of ACC we detected a significant effect of diagnosis (P<0.04) with GFAP mRNA levels decreased in individuals with schizophrenia and bipolar disorder as compared with normal controls. In the gray matter there was a significant effect of layer (P<0.01) with the highest levels of GFAP mRNA in layer VI in all groups. As in the white matter, the mean GFAP mRNA levels were decreased in individuals with schizophrenia and bipolar disorder as compared with the unaffected controls, however the difference failed to reach statistical significance. Thus, astrocytes positive for GFAP may contribute to the decrease in glial density previously described in subjects with major mental illness, however the relative contribution of astrocytes may vary with diagnosis.

MHC class I, beta2 microglobulin, and the INF-gamma receptor are upregulated in aged motoneurons

During aging, spinal cord motoneurons show characteristic changes including the loss of afferent boutons, a selective process that associates with gliosis and behavioral motor impairment. Evidence suggests that the major histocompatibility complex Class I (MHC I) system may be involved in synaptic plasticity of neurons during development and regeneration. In search of a mechanism governing senescent changes in synaptic connectivity, we report evidence for increased expression of MHC I and beta2 microglobulin (beta2M) in motoneurons and glial-like profiles of 30-month-old rats. The regulatory signal(s) for MHC I expression in normal neurons remains unresolved but among tentative molecules are cytokines such as interferon-gamma (INF-gamma) and tumor necrosis factor alpha (TNF-alpha). Interestingly, aged motoneurons, overlapping with those showing increased levels of MHC I, contained increased levels of INF-gamma receptor message. INF-gamma mRNA was detected at low levels in most (8/9) of the aged spinal cords but only infrequently (2/9) in young adult spinal cords; however, the cellular localization of INF-gamma mRNA could not be determined. Our data also indicates that TNF-alpha is upregulated in the senescent spinal cord but that TNF-alpha immunoreactive protein does not associate with motoneurons. We report evidence for an increased expression of MHC I and beta2M in senescent spinal motoneurons and discuss the possibility that this regulation associates with INF-gamma or changes in neurotrophin signaling and neuron activity in senescence.

Developmental changes of parameters for astrogliosis during cultivation of purified cerebral astrocytes from newborn rats

Astrogliosis is a common phenomenon seen in most neuropathological changes of the central nervous system. Several in vitro models have been used to study the mechanisms and conditions for the induction of astrogliosis, however many do not take into account that the metabolic and structural characteristics of astrocytes change with time in culture. Thus, it appears difficult to attribute changes of, e.g., GFAP to the normal change in vitro as opposed to additional changes due to an astrogliotic reaction. The present study was therefore undertaken to characterize these developmental changes in purified astroglial secondary cultures during cultivation to provide a basis for further investigations of astrogliosis in vitro. During 6 weeks of cultivation (3-43 days) GFAP (ELISA) increased much more (22-fold) than the cell number (2.5-fold) and the total protein (3.5-fold). The GFAP/protein ratio increased during the first 4 weeks of cultivation and reached a plateau thereafter, which was accompanied by a significant increase of GFAP mRNA (Northern blot). At the ultrastructural level (transmission electron microscopy) gliofilaments in the perinuclear region as well as in the cell processes of 4-day-old astrocytes showed a dispersed pattern, whereas an accumulation of gliofilaments was found in 39-day-old cells, which formed large aggregated bundles localized mostly in the cell processes. Our results show that in vitro astrocytes undergo developmental changes in their accumulation of GFAP and intermediate filaments which reach a stable steady state after 4 weeks in culture. These 'normal' developmental changes will have to be taken into account, when experiments with variations of the level of GFAP are performed. Stable culture conditions for experimentation appear to be present after 4 weeks in culture.

Dose-dependent decrease in glial fibrillary acidic protein-immunoreactivity in rat cerebellum after lifelong ethanol consumption

The effects of aging and lifelong ethanol consumption on astrocytic morphology and glial fibrillary acidic protein-immunoreactivity (GFAP-IR) in the cerebellar vermis obtained from ethanol-preferring Alko, Alcohol (AA) rats were analyzed by using computer-assisted image analysis. The ethanol-consuming animals (both male and female) were given ethanol (10%-12%, vol./vol.) as the only available fluid for 21 months (3-24 months), whereas the young (3 months) and the old (24 months) controls received water. In the male rats, but not in the female rats, an age-related decrease in GFAP-IR was found in folia II, VII, and X of the molecular layer, and in turn, an age-related increase was found in folium X of the granular layer, indicating opposite changes in GFAP-IR for male rats due to aging in adjacent brain regions. In the female rats, 21 months of daily average ethanol consumption of 6.6 g/kg resulted in decreased GFAP-IR in folium VII of the molecular layer, and the decrease in cerebellar GFAP-IR correlated with the average daily ethanol intake (r=-.886, P=.019) when folia II, IV, VII, and X were analyzed together. No effect of ethanol on GFAP-IR was detected in the granular layer or in the central white matter of the female rats. There was no change in GFAP-IR in any of the three cerebellar layers of the male rats with average daily ethanol consumption of 3.2 g/kg. These results indicate that the Bergmann glial fibers are the GFAP-expressing structures of the cerebellum most sensitive to moderate-to-heavy chronic ethanol exposure and that this effect is dose dependent.

Astrocytic hypertrophy and altered GFAP degradation with age in subcortical white matter of the rhesus monkey

Reactive astrocytosis is a well known phenomenon that occurs in the normal aging process of the brain. While many studies indicate astrocytic hypertrophy and glial fibrillary acidic protein (GFAP) content increase with age in the hippocampal formation of certain animal models, it is unclear whether these findings are generalizable to the primate and to other areas of the brain. In this study, we quantitatively assessed age-related changes in astrocytic cell size and density in a rhesus monkey model of normal aging. By GFAP immunohistochemistry, we observed an increase in GFAP(+) cell size but not density in all subcortical white matter areas of the frontal, temporal, and parietal cortices. No significant increases in astrocyte hypertrophy were observed in any gray matter area examined. In addition, Western blotting experiments showed increases in total and degraded GFAP content with age, suggesting altered degradation and possibly production of GFAP occur with age.

Calorie restriction modulates age-dependent changes in the retinas of Brown Norway rats

The present study examined the effect of a 40% reduction in caloric intake (CR) versus ad libitum (AL) feeding on retinal aging. CR- and AL-fed Brown Norway (BN) rats were obtained at 12, 24 and 30 months of age from the National Center for Toxicological Research (NCTR). Age-dependent declines in outer nuclear layer (ONL=photoreceptor) cell densities, ONL height, inner nuclear layer (INL) cell densities, and thicknesses of the inner retina and whole retina were quantified in thick sections at six loci across the circumference of the sensory retina (four peripheral, two central). Data were analyzed by repeated measures, general linear models. Aging in both diet groups was associated with declines in ONL cell density, ONL height, peripheral INL cell density and total retinal thickness (P< or =0.05). However, ONL cell densities, ONL height and retinal thickness were significantly greater in the CR versus AL diet group at all three ages (P< or =0.005). CR was also associated with a trend for greater peripheral INL cell density (P=0.06) and with greater INL thickness at 30 months (Bonferroni P=0.03). Elevated ONL cell densities in the CR-12 cohort relative to the AL-12 cohort could be explained by diet-associated differences in retinal length, i.e. delayed retinal growth in response to CR. Enhanced ONL cell density, ONL height, INL cell density, INL thickness and total retinal thickness in the CR-30 cohort appear to be as a result of reduced rates of retinal cell loss between 24 and 30 months. However, the protective effect of CR in retinas of older animals may also reflect the initial growth-associated enhancements which were observed in 12 month-old animals. The rat retina may provide a useful model for elucidating the neuroprotective mechanism(s) of CR.

Behavioural and glial changes in old rats following environmental enrichment

The effects of enriched environment on short-term memory for event durations and on astrocytes (cell density, cell area and % of GFAP immunoreactivity) in hippocampus (Hi), frontal cortex (FC) and corpus callosum (CC) were analysed in old rats housed from weaning to the end of behavioural testing (23 months) either in standard (SC) or in enriched (EC) conditions and in young adults (5 months) all housed in SC. Old SC and EC and young SC rats trained (for 2 months) or not, in a Symbolic Delayed Matching to Sample Task, had to discriminate and remember two (2- and 10-s) signals after short retention intervals. Results confirm the aging-related acquisition and memory deficit. EC reduced the slowness of acquisition, reversed the short-term memory deficit and promoted the retention of the short signal (choose short effect). Old SC naive rats had many hypertrophied astrocytes with long processes in Hi and CC while old EC rats had decreased astrocytes number and size. The behavioural testing resulted in young adult SC rats in Hi and CC, in increased astrocytes number, size and GFAP% and in their decrease in old SC rats. EC and testing have additive effects (very low astrocytes number, size and GFAP%) to compensate for the aging-induced gliosis, mostly in Hi.

Glial reaction in aging and Alzheimer's disease

It is well-established that glial cells play an important role during injury and neurodegenerative processes in the central nervous system. In normal aging, no global glia proliferation is found morphologically, but reactive gliosis has been described in specific areas of the limbic system and neocortex that undergo selective neuronal or synaptic degeneration in nondemented elderly persons. In addition, there is an age-associated increase in the metabolic turnover of cellular proteins, such as glial fibrillary acidic protein, in human brain tissue, even without detectable signs of neurodegeneration. In contrast to the relatively moderate overall glial changes in normal aging, the close association of activated astrocytes and microglial cells with neuritic plaques and cells undergoing neurofibrillary degeneration in Alzheimer's disease (AD), the expression of receptors for complement by glial cells, and the release of soluble cytokines strongly suggest that inflammatory processes may play an important part in the complex pathophysiological interactions that occur in AD. Understanding the role of glia in age-associated neurodegenerative disorders may provide new insights into the neurobiology of glia-neuronal interaction and may allow the development of strategies to alter the disease process. This review aims to summarize some of the important aspects of glial cells in aging and dementia.

Stereological analysis of astrocyte and microglia in aging mouse hippocampus

Recent evidence suggests neuroglia-mediated inflammatory mechanisms may stimulate neurodegenerative processes in mammalian brain during aging. To test the hypothesis that the number of microglia and astrocytes increase in the hippocampus during normal aging, unbiased stereological techniques were used to estimate total cell number in hippocampal subregions (CA1, dentate gyrus and hilus) of male C57BL/6J mice of different ages: 4-5 months, 13-14 months and 27-28 months. Immunocytochemical visualization for microglia and astrocytes were via Mac-1 and GFAP antibody, respectively. Estimates of total microglia and astrocyte number were assessed using the optical fractionator. No statistically significant age differences were found in the numbers of microglia or astrocytes in the hippocampal regions sampled. These findings suggest that age-related increases in the total numbers of hippocampal microglia and astrocytes is not causal for observed age-related increases in cytokine response.

Neuronal ceroid lipofuscinosis (nclf), a new disorder of the mouse linked to chromosome 9

The neuronal ceroid lipofuscinoses (NCLs) comprise a set of at least 6 distinct human and an unknown number of animal diseases characterized by storage of proteolipids in lysosomes of many cell types. By unknown mechanisms, this accumulation leads to or is associated with severe neuronal and retinal degeneration. The genes for 3 human NCLs, infantile, late infantile, and juvenile, have been cloned. The first murine form of NCL, the motor neuron degeneration (mnd) mouse, has been described and mapped to proximal Chromosome 8. Here we describe a second genetic variant of NCL in the mouse, neuronal ceroid lipofuscinosis, nclf. These mice exhibited a phenotype that was almost exactly the same as that observed in mnd/mnd mice. Homozygous nclf mice developed progressive retinal atrophy early in life and become paralyzed at around 9 months of age. They accumulated luxol fast blue staining material in cytoplasm of neurons and many other cell types. Ultrastructurally, affected lysosomes had a "finger print pattern" with membranous material arranged in "pentalaminar" patterns. Affected mice developed severe cerebral gliosis in late stages of their disease. They also had severe Wallerian degeneration of long tracts in spinal cord and brain stem, lesions that accounted for the distinctive upper motor neuron signs displayed by both nclf/nclf and mnd/mnd mice. By crossing nclf/nclf mice with CAST/Ei mice, linkage analysis of nclf with respect to SSLP markers was performed, showing that nclf is located on Chromosome 9 between D9Mit164 and D9Mit165, in a region that is homologous with human Ch 15q21, where the gene for one variant of late infantile NCL, CLN6, recently has been mapped. The genes for two proteolipids known to be stored in lysosomes of animals and people with NCL were also mapped in this study and found not to map to the mnd or nclf loci nor to any mouse locus homologous to any known human NCL disease locus.

Murine models of brain aging and age-related neurodegenerative diseases

In the past, structural changes in the brain with aging have been studied using a variety of animal models, with rats and nonhuman primates being the most popular. With the rapid evolution of mouse genetics, murine models have gained increased attention in the neurobiology of aging. The genetic contribution of age-related traits as well as specific mechanistic hypotheses underlying brain aging and age-related neurodegenerative diseases can now be assessed by using genetically-selected and genetically-manipulated mice. Against this background of increased demand for aging research in mouse models, relatively few studies have examined structural alterations with aging in the normal mouse brain, and the data available are almost exclusively restricted to the C57BL/6 strain. Moreover, many older studies have used quantitative techniques which today can be questioned regarding their accuracy. Here we review the state of knowledge about structural changes with aging in outbred, inbred, genetically-selected, and genetically-engineered murine models. Moreover, we suggest several new opportunities that are emerging to study brain aging and age-related neurodegenerative diseases using genetically-defined mouse models. By reviewing the literature, it has become clear to us that in light of the rapid progress in genetically-engineered and selected mouse models for brain aging and age-related neurodegenerative diseases, there is a great and urgent need to study and define morphological changes in the aging brain of normal inbred mice and to analyze the structural changes in genetically-engineered mice more carefully and completely than accomplished to date. Such investigations will broaden knowledge in the neurobiology of aging, particularly regarding the genetics of aging, and possibly identify the most useful murine models.

Aging is associated with divergent effects on Nf-L and GFAP transcription in rat brain

We studied the effects of advancing age on the expression of several proteins important in the structure and function of the nervous system. Brains of young (3 month), middle-aged (13 month), and old (29 month) male Fischer 344 rats were examined. Run-on transcription and Northern blot hybridizations were used to determine gene-specific transcription rates and mRNA levels, respectively. With advancing age, there was a decrement in the transcription rate and mRNA levels for neurofilament-light subunit (Nf-L), but an increment in the transcription rate and mRNA levels for glial fibrillary acidic protein (GFAP). Proteolipid protein (PLP) mRNA levels were attenuated between 3 and 13 months of age, whereas amyloid precursor protein (APP) mRNA levels were attenuated in the middle-aged but not the old animals. Transcription rates for alpha-actin and fos, and mRNA levels for alpha-actin, were unaffected. These observations indicate divergent transcriptional regulation of several genes, notably Nf-L and GFAP, in the aging mammalian forebrain.

Age-induced hypertrophy of astrocytes in rat supraoptic nucleus: a cytological, morphometric, and immunocytochemical study

BACKGROUND

In the adult rat, neuron-astroglia interactions in the supraoptic nucleus (SON) are characterized by the structural and functional plasticity of astrocytes in response to several physiological and experimental conditions. This study has analyzed the plasticity of the supraoptic nucleus astrocytes in response to the age-induced changes in neuronal activity.

METHODS

The study was performed in 5-, 12-, 18- and 24-month-old rats. The cytology and organization of astrocytes in the SON were examined using glial fibrillary acidic and vimentin immunocytochemistry and ultrastructural and morphometric analysis.

RESULTS

No significant age-related variations in the total number of neurons and astrocytes in the SON were detected, although a few degenerating neurons were found in old rats. An age-dependent increase in GFAP immunoreactivity was observed at the ventral glial lamina, perivascularly and between neuronal perikarya. Vimentin overexpression was also detected in ventral lamina astrocytes with advancing age. At the cell nucleus level, we observed an age-associated increase in nuclear size and in the number of coiled bodies, nuclear bodies, and "cleared" nucleoplasmic areas, as well as changes in the nucleolar organization. At the cytoplasmic level, characteristic ultrastructural features in astrocytes of old rats were the hypertrophy of intermediate filament bundles and the formation of an extensive network of Golgi stacks interlinked by tubulovesicular elements. Glial filaments were often associated with the nuclear envelope and polyribosomes.

CONCLUSIONS

The increased GFAP and vimentin immunoreactivity and the morphometric and cytological changes in rat SON astrocytes may reflect a sustained upregulation of cellular activity with age, resulting in hypertrophy of glial perikarya and cell processes. Several factors that are known to influence the expression of the astrocytic phenotype, such as signals produced by degenerating neurons and activated microglia, as well as variations in neuronal activity are considered possible causes of the age-associated changes in SON astrocytes.

Increased glial fibrillary acidic protein (GFAP) levels in the brains of transgenic mice expressing the bovine growth hormone (bGH) gene

Transgenic mice, expressing the gene for bovine growth hormone (bGH), exhibit increased body size, reduced reproductive capacity, and high basal levels of several hormones including corticosterone. Their shortened life span may be indicative of accelerated aging. As prominent astrogliosis of the CNS accompanies aging in rodents, bGH transgenic mice were examined for astrogliosis, as quantified by an ELISA for the astrocyte-localized protein, glial fibrillary acidic protein (GFAP). Transgenic mice were produced by mating C57BL/6 x C3H F1 hybrid females with male descendants of animals produced by microinjection of fertilized eggs with phosphoenolpyruvate carboxykinase (PEPCK)/bGH-hybrid gene. Transgenic mice (approximately 3.5 and approximately 12 months of age) weighed significantly more than same age or older (approximately 20 month) controls. Most of their internal organs, including the heart, kidneys, adrenals, liver, and spleen, were also heavier. In contrast, the thymus was heavier only in the younger transgenic mice. Serum corticosterone was highest in the older transgenic mice. A small but significant increase in whole brain, cortex, and cerebellar weight, relative to controls and the older transgenic mice, was found in the younger transgenic mice. Control mice exhibited large, significant age-related increases in GFAP. Increases of 35, 70, 68, 89, 79, and 95% for cortex, cerebellum, striatum, hippocampus, midbrain, and brain stem, respectively, were found when comparing the oldest (approximately 20 months) control mice to the youngest (approximately 3.5 months). In contrast, in the olfactory bulbs and the hypothalamus there were no age-related changes in the levels of GFAP in control mice. Transgenic mice (approximately 3.5 months) had significantly elevated GFAP levels relative to the same-age controls in all brain areas examined. In some brain areas, the GFAP levels found in the younger transgenic mice were equivalent to those found in the oldest controls. No differences between controls and transgenics were found in tyrosine hydroxylase protein levels of striatum or hypothalamus. The elevated GFAP levels of transgenic mice may reflect increased neural damage due to accelerated aging processes or damage associated with high circulating levels of bGH or corticosterone. Alternatively, the increased expression of GFAP in the transgenic mice may reflect altered regulation of GFAP rather than an increase signaled by neural damage.

Food restriction delays the age-related increase in GFAP mRNA in rat hypothalamus

Astrogliosis with advancing age is correlated with increased expression of glial fibrillary acidic protein (GFAP). Hypothalamic GFAP mRNA prevalence was determined in male F344 rats of different ages that were fed ad lib (AL) and compared with that of rats that were food-restricted (FR) to 60% of AL levels. Hypothalamic GFAP mRNA increased 3-fold at 24 to 25 months in AL rats compared with 3 and 6 month groups. There were no differences in GFAP mRNA levels between AL and FR rats from 3 to 18 months. However, GFAP mRNA was significantly lower in FR than in AL rats at 24 to 25 months; FR rats reached the level of GFAP mRNA in 24 to 25 months AL rats by 33 months. Hypothalamic glutamine synthetase mRNA also increased with age in both dietary groups but did not differ between dietary groups at any age. The observation that FR delays the increased expression of GFAP in the hypothalamus during aging lends support to the hypothesis that upregulation of GFAP mRNA is a biomarker of brain aging.

Increases of glial fibrillary acidic protein in the aging female mouse brain

Age-related increases of the astrocyte marker, glial fibrillary acidic protein (GFAP), were further resolved by in situ hybridization and immunocytochemistry in female C57BL/6J mice. The age groups represented the major stages of reproductive aging: young (5 months), middle-age (18 months), and old (23 and 26 months). GFAP mRNA and protein showed generalized increases in old mice. Major white fiber tracts, such as the corpus callosum, fimbria, stria terminalis, and optic tract, showed increased GFAP immunostaining and mRNA. Gray matter showed robust > or = twofold increases in GFAP mRNA with age, especially in the thalamus and hypothalamus, areas that expressed little GFAP in the young. These generalized age-related increases of GFAP in many brain regions imply the existence of a widespread stimulus for increased activity of astrocytes during aging.

Age-related increases in glial fibrillary acidic protein do not show proportionate changes in transcription rates or DNA methylation in the cerebral cortex and hippocampus of male rats

Age-related increases in the expression of glial fibrillary acidic protein (GFAP) in many brain regions are observed in short- and long-lived mammals. Possible genomic mechanisms for the increase of GFAP mRNA and protein were studied in the hippocampus and cortex of male F344 rats and a longer-lived hybrid F1 (F344 x Brown Norway). No age-related changes were found in the extent of cytosine methylation at 19 CpG sites in the 5'-upstream GFAP promoter and in exon 1. With the nuclear runon assay, no change was found in the transcription rate of GFAP in the cerebral cortex or hippocampus. Thus, age-related increases in GFAP are not associated with proportionate changes in transcription rates or DNA methylation. However, the transcription of glutamine synthetase was increased by about 60%. These findings contrast with age-related loss of bulk tissue DNA methylation and decreased transcription rates of other genes reported in non-neural tissues.

Ultrastructural changes in brain parenchyma during normal aging and in animal models of aging

During aging, the brain parenchyma of animals and humans share many similarities, both in the gray and the white matter. Unfortunately, until now, neither aged animals nor animal models reproduce the two hallmarks of aging of the human brain: senile plaques and tangles. Therefore, observations performed on animals are limited to some aspects of the involutive process which affects brain parenchyma during aging and their appropriateness to the human situation. One striking aspect concerns the occurrence of vacuolated necrotic cells whose number increases with advancing age. These cells can constitute markers of the brain involutive process and they characterize, both in animal and human, the more vulnerable areas of the brain affected by the neuronal rarefaction. Experimental animal models can be used to study the various conditions which sustain the cell survival and to determine, at the cellular level, the factors leading the brain parenchyma to an irreversible state of degradation.