The pathogenesis of incipient Alzheimer's disease (AD) has been resistant to analysis because of the complexity of AD and the overlap of its early-stage markers with normal aging. Gene microarrays provide new tools for addressing complexity because they allow overviews of the simultaneous activity of multiple cellular pathways. However, microarray data interpretation is often hindered by low statistical power, high false positives or false negatives, and by uncertain relevance to functional endpoints. Here, we analyzed hippocampal gene expression of nine control and 22 AD subjects of varying severity on 31 separate microarrays. We then tested the correlation of each gene's expression with MiniMental Status Examination (MMSE) and neurofibrillary tangle (NFT) scores across all 31 subjects regardless of diagnosis. These well powered tests revealed a major transcriptional response comprising thousands of genes significantly correlated with AD markers. Several hundred of these genes were also correlated with AD markers across only control and incipient AD subjects (MMSE > 20). Biological process categories associated with incipient AD-correlated genes were identified statistically (ease program) and revealed up-regulation of many transcription factor/signaling genes regulating proliferation and differentiation, including tumor suppressors, oligodendrocyte growth factors, and protein kinase A modulators. In addition, up-regulation of adhesion, apoptosis, lipid metabolism, and initial inflammation processes occurred, and down-regulation of protein folding/metabolism/transport and some energy metabolism and signaling pathways took place. These findings suggest a new model of AD pathogenesis in which a genomically orchestrated up-regulation of tumor suppressor-mediated differentiation and involution processes induces the spread of pathology along myelinated axons.

https://www.pnas.org/content/pnas/101/7/2173.full.pdf

Incipient Alzheimer's disease: Microarray correlation analyses reveal major transcriptional and tumor suppressor responses

Steroid hormones, particularly estrogens and glucocorticoids, may play roles in the pathogenesis of neurodegenerative disorders, but their mechanisms of action are not known. We report that estrogens protect cultured hippocampal neurons against glutamate toxicity, glucose deprivation, FeSO4 toxicity, and amyloid beta-peptide (A beta) toxicity. The toxicity of each insult was significantly attenuated in cultures pretreated for 2 h with 100 nM-10 microM 17 beta-estradiol, estriol, or progesterone. In contrast, corticosterone exacerbated neuronal injury induced by glutamate, FeSO4, and A beta. Several other steroids, including testosterone, aldosterone, and vitamin D, had no effect on neuronal vulnerability to the different insults. The protective actions of estrogens and progesterone were not blocked by actinomycin D or cycloheximide. Lipid peroxidation induced by FeSO4 and A beta was significantly attenuated in neurons and isolated membranes pretreated with estrogens and progesterone, suggesting that these steroids possess antioxidant activities. Estrogens and progesterone also attenuated A beta- and glutamate-induced elevation of intracellular free Ca2+ concentrations. We conclude that estrogens, progesterone, and corticosterone can directly affect neuronal vulnerability to excitotoxic, metabolic, and oxidative insults, suggesting roles for these steroids in several different neurodegenerative disorders.

Estrogens Attenuate and Corticosterone Exacerbates Excitotoxicity, Oxidative Injury, and Amyloid β‐Peptide Toxicity in Hippocampal Neurons

Glucocorticoid action on target tissues is determined by the density of “nuclear” receptors and intracellular metabolism by the two isozymes of 11β-hydroxysteroid dehydrogenase (11β-HSD) which catalyze interconversion of active cortisol and corticosterone with inert cortisone and 11-dehydrocorticosterone. 11β-HSD type 1, a predominant reductase in most intact cells, catalyzes the regeneration of active glucocorticoids, thus amplifying cellular action. 11β-HSD1 is widely expressed in liver, adipose tissue, muscle, pancreatic islets, adult brain, inflammatory cells, and gonads. 11β-HSD1 is selectively elevated in adipose tissue in obesity where it contributes to metabolic complications. Similarly, 11β-HSD1 is elevated in the ageing brain where it exacerbates glucocorticoid-associated cognitive decline. Deficiency or selective inhibition of 11β-HSD1 improves multiple metabolic syndrome parameters in rodent models and human clinical trials and similarly improves cognitive function with ageing. The efficacy of inhibitors in human therapy remains unclear. 11β-HSD2 is a high-affinity dehydrogenase that inactivates glucocorticoids. In the distal nephron, 11β-HSD2 ensures that only aldosterone is an agonist at mineralocorticoid receptors (MR). 11β-HSD2 inhibition or genetic deficiency causes apparent mineralocorticoid excess and hypertension due to inappropriate glucocorticoid activation of renal MR. The placenta and fetus also highly express 11β-HSD2 which, by inactivating glucocorticoids, prevents premature maturation of fetal tissues and consequent developmental “programming.” The role of 11β-HSD2 as a marker of programming is being explored. The 11β-HSDs thus illuminate the emerging biology of intracrine control, afford important insights into human pathogenesis, and offer new tissue-restricted therapeutic avenues.

11β-Hydroxysteroid Dehydrogenases: Intracellular Gate-Keepers of Tissue Glucocorticoid Action

Gene expression microarrays provide a powerful new tool for studying complex processes such as brain aging. However, inferences from microarray data are often hindered by multiple comparisons, small sample sizes, and uncertain relationships to functional endpoints. Here we sought gene expression correlates of aging-dependent cognitive decline, using statistical profiling of gene microarrays in well powered groups of young, mid-aged, and aged rats (n = 10 per group). Animals were trained on two memory tasks, and the hippocampal CA1 region of each was analyzed on an individual microarray (one chip per animal). Aging- and cognition-related genes were identified by testing each gene by ANOVA (for aging effects) and then by Pearson's test (correlating expression with memory). Genes identified by this algorithm were associated with several phenomena known to be aging-dependent, including inflammation, oxidative stress, altered protein processing, and decreased mitochondrial function, but also with multiple processes not previously linked to functional brain aging. These novel processes included downregulated early response signaling, biosynthesis and activity-regulated synaptogenesis, and upregulated myelin turnover, cholesterol synthesis, lipid and monoamine metabolism, iron utilization, structural reorganization, and intracellular Ca2+ release pathways. Multiple transcriptional regulators and cytokines also were identified. Although most gene expression changes began by mid-life, cognition was not clearly impaired until late life. Collectively, these results suggest a new integrative model of brain aging in which genomic alterations in early adulthood initiate interacting cascades of decreased signaling and synaptic plasticity in neurons, extracellular changes, and increased myelin turnover-fueled inflammation in glia that cumulatively induce aging-related cognitive impairment.

https://www.jneurosci.org/content/jneuro/23/9/3807.full.pdf

Gene microarrays in hippocampal aging: statistical profiling identifies novel processes correlated with cognitive impairment.

Prolonged treatment with stress levels of corticosterone has been reported to produce changes in the hippocampus. In the experiments reported here, we examined for functional and morphological consequences of this treatment. First, young adult or mid-aged male Long-Evans rats were treated for either 1 or 3 months with corticosterone, at a dose sufficient to mimic the elevated hormone levels observed following exposure to mild stress. Two weeks following the termination of treatment, the animals were tested in the Morris water maze to assess spatial learning. No behavioral deficits were observed after 1 month of treatment. A 3 month treatment period also had no effect in young rats, but produced a learning impairment in the mid-aged rats. We then examined whether the effect of elevated corticosterone in mid-aged animals could be produced by a physiological stressor. Mid-aged rats were maintained for 6 months under conditions of low or high social stress. Six months of exposure to high social stress produced significant spatial learning impairments in the Morris water maze. These effects were absent in high social stress animals that had been previously adrenalectomized (with low-level corticosterone replacement), suggesting that elevated glucocorticoid levels mediate the effects of stress on spatial memory in older animals. In a final experiment, mid-aged rats were treated with corticosterone at levels that mimicked those naturally occurring at the diurnal peak (medium-B: 12–17 micrograms/dl) or in response to stress (high-B: 25–32 micrograms/dl). Only rats exposed to high levels of corticosterone demonstrated impaired performance in the Morris water maze.(ABSTRACT TRUNCATED AT 250 WORDS)

https://www.jneurosci.org/content/jneuro/15/1/61.full.pdf

Enduring effects of chronic corticosterone treatment on spatial learning, synaptic plasticity, and hippocampal neuropathology in young and mid-aged rats

This paper reviews evidence that brain aging and Alzheimer's disease (AD) are somehow closely related and that the hippocampus (CA1) is highly vulnerable to cell loss under both conditions. In addition, two current lines of evidence on the mechanisms of hippocampal cell loss with aging are considered, including studies of neuronal calcium dysregulation and studies of cumulative glucocorticoid (GC) neurotoxicity. Moreover, recent electrophysiological studies have shown that excess glucocorticoid activation of hippocampal neurons increases the influx of calcium through voltage-activated calcium channels. Second messenger systems may mediate the steroid modulation of calcium channels. Therefore, it is hypothesized that excess glucocorticoid activation and neuronal calcium dysregulation may be two phases of a single process that increases the susceptibility of neurons to neurodegeneration during aging and Alzheimer's disease.

D S Kerr

Papers

Mechanisms of neuronal death in brain aging and Alzheimer's disease: role of endocrine-mediated calcium dyshomeostasis.