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Glucocorticoid-Dependent Hippocampal Transcriptome in Male Rats: Pathway-Specific Alterations with Aging

ABSTRACT: Although glucocorticoids (GCs) are known to exert numerous effects in the hippocampus, their chronic regulatory functions remain poorly understood. Moreover, evidence is inconsistent regarding the longstanding hypothesis that chronic GC exposure promotes brain aging/Alzheimer's disease. Here, we adrenalectomized male F344 rats at 15-months-of-age, maintained them for 3 months with implanted corticosterone (CORT) pellets producing low or intermediate (glucocorticoid-receptor (GR)-activating) blood levels of CORT, and performed microarray/pathway analyses in hippocampal CA1. We defined the chronic GC-dependent transcriptome as 393 genes that exhibited differential expression between Intermediate- and Low-CORT groups. Short-term CORT (4 days) did not recapitulate this transcriptome. Functional processes/pathways overrepresented by chronic CORT-upregulated genes included learning/plasticity, differentiation, glucose metabolism and cholesterol biosynthesis, whereas processes overrepresented by CORT-downregulated genes included inflammatory/immune/glial responses and extracellular structure. These profiles indicate that GCs chronically activate neuronal/metabolic processes while coordinately repressing a glial axis of reactivity/inflammation. We then compared the GC-transcriptome with a previously-defined hippocampal aging transcriptome, revealing a high proportion of common genes. Although CORT and aging moved expression of some common genes in the same-direction, the majority were shifted in opposite directions by CORT and aging (e.g., glial inflammatory genes downregulated by CORT are upregulated with aging). These results contradict the hypothesis that GCs simply promote brain aging, and also suggest that the opposite-direction shifts during aging reflect resistance to CORT regulation. Therefore, we propose a new model in which aging-related GC resistance develops in some target pathways while GC overstimulation develops in others, together generating much of the brain aging phenotype.

ORGANISM(S): Rattus norvegicus

PROVIDER: GSE37618 | GEO | 2013/07/22

SECONDARY ACCESSION(S): PRJNA162371

REPOSITORIES: GEO

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Project description:Although glucocorticoids (GCs) are known to exert numerous effects in the hippocampus, their chronic regulatory functions remain poorly understood. Moreover, evidence is inconsistent regarding the longstanding hypothesis that chronic GC exposure promotes brain aging/Alzheimer's disease. Here, we adrenalectomized male F344 rats at 15-months-of-age, maintained them for 3 months with implanted corticosterone (CORT) pellets producing low or intermediate (glucocorticoid-receptor (GR)-activating) blood levels of CORT, and performed microarray/pathway analyses in hippocampal CA1. We defined the chronic GC-dependent transcriptome as 393 genes that exhibited differential expression between Intermediate- and Low-CORT groups. Short-term CORT (4 days) did not recapitulate this transcriptome. Functional processes/pathways overrepresented by chronic CORT-upregulated genes included learning/plasticity, differentiation, glucose metabolism and cholesterol biosynthesis, whereas processes overrepresented by CORT-downregulated genes included inflammatory/immune/glial responses and extracellular structure. These profiles indicate that GCs chronically activate neuronal/metabolic processes while coordinately repressing a glial axis of reactivity/inflammation. We then compared the GC-transcriptome with a previously-defined hippocampal aging transcriptome, revealing a high proportion of common genes. Although CORT and aging moved expression of some common genes in the same-direction, the majority were shifted in opposite directions by CORT and aging (e.g., glial inflammatory genes downregulated by CORT are upregulated with aging). These results contradict the hypothesis that GCs simply promote brain aging, and also suggest that the opposite-direction shifts during aging reflect resistance to CORT regulation. Therefore, we propose a new model in which aging-related GC resistance develops in some target pathways while GC overstimulation develops in others, together generating much of the brain aging phenotype. Forty male Fischer 344 rats aged 14-15 months (late mid-age) were either sham operated (n=10) or adrenalectomized (n=30) at Harlan, Inc. (Indianapolis) and shipped to University of Kentucky 7-10 d after surgery. Animals were allowed to acclimate for 2 weeks after receipt prior to pellet-implant and were provided with food and water ad lib. Adrenalectomized (ADX) animals were maintained on drinking water containing isotonic saline with 1% sucrose, and also given supplemental feed mash (standard chow mixed with water and warmed) and apple chunks, as well as subcutaneous injections of 1.25 mg corticosterone sulfate (Solu-DeltaCort, Pfizer). During pellet-implant surgery, subjects were anesthetized with isoflurane, placed on a warming pad, and a 3 cm2 area on the animal's flank was shaved and sterilized. A 1-cm incision was made through the skin to form a subcutaneous pocket. Each sham animal received an inert pellet. ADX animals were divided into two groups, low-dose animals (n = 20) received one 25 mg CORT pellet and intermediate-dose animals (n = 10) received one 200 mg CORT pellet (slow release pellets, Innovative Research of America, Sarasota, FL). Incisions were closed with wound clips (removed after 7-10 days). Following implant surgery, subjects were housed singly for 3 months prior to euthanasia at age 18 months. Low-dose animals continued to receive saline/ sucrose drinking water, but other supplementation (food mash, Solu-Delta CORT, apple chunks) was discontinued. One ADX animal was removed from the study for declining health. During the twelfth week, just prior to study termination, half of the ADX low-dose CORT subjects were given a short-term, high-dose regimen consisting of 4 daily IP injections of 5mg/d CORT (considered a relatively high dose- Makino et al., 1995). All animals were killed between 8 and 10 AM on the last two days of the study. After deep CO2 anesthesia and decapitation, trunk blood was collected for CORT radioimmunoassay and hippocampal CA1 brain tissue was dissected as described previously (Blalock et al., 2003) for microarray analysis (see below). Final treatment groups consisted of: 1) Sham (intact) control, 2) Low-CORT (ADX), 3) Intermediate-CORT (ADX), and 4) Low-CORT (ADX) + short-term high-dose injected (n=9-10/group).

Project description:Purpose: we sought to search for genes that are transcribed or repressed in a glucocorticoid (GC)-dependent manner in a hypothalamic region containing the paraventricular nucleus of the hypothalamus (PVH), thereby unraveling mechanisms underlying GC-induced neuroendocrine and autonomic regulation. Methods: we employed unbiased quantification of the abundance of DNA transcripts provided by RNA-seq in the PVH region of male rats and identified genes whose expression is enhanced or suppressed by the 12-day exposure to a high dose of corticosterone (Cort) by Cort implant (12-day high Cort) or deprivation of corticoids for 7 days [7-day adrenalectomy (ADX)] (Experiment 1), as well as by acute Cort increase (Experiment 2). Sham-ADX animals served as controls in Experiment 1. In Experiment 2, rats underwent ADX and supplemented with Cort in drinking water. The animals were decapitated at 1 h [acute Cort (1) 1 h] or 3 h [acute Cort (1) 3 h] following injection of HBC Cort (1 mg), a dose shown to elicit plasma Cort surge comparable to that during acute stress. A group of rats without Cort injection served as controls (ADX + Cort suppl). Differential expression was calculated as fold-changes in gene expression [measured as fragments per kilobase of exon per million reads mapped (FPKM)]. The Benjamini & Hochberg method was applied to correct the unavoidable false positive data ensued from the multiple tests, and a q-value was calculated as the false discovery rate-adjusted p-value. The q-value < 0.05 was used as the cut-off criterion. Pearson’s correlation analysis was carried out for grouping genes that showed similar expression patterns. qRT–PCR validation was performed using Power SYBR Green PCR mix. Semi-quantitative in situ hybridization was carried out using 35S-labeled cRNA probes. Results: some canonical GC target genes, among others of a broad functional spectrum, were upregulated prominently by 12-day high Cort. Crh was upregulated or downregulated most prominently by either 7-day ADX or 12-day high Cort, emphasizing the recognized feedback effects of GC on the hypothalamic-pituitary-adrenal (HPA) axis. Concomitant changes in Avp and Aplnr expression are likely to contribute to HPA repression. In keeping with the pleotropic cellular actions of GCs, 7-day ADX downregulated numerous genes of a broad functional spectrum. The transcriptome response signature differed markedly between acute Cort injection and 12-day high Cort. Remarkably, 6 immediate early genes, Bsx, Egr1, Fos, Fosb, Junb, and Nr4a3, were upregulated at 1 h after Cort injection, and the changes were confirmed by qRTPCR and semi-quantitative in situ hybridization. Conclusions: the present study uncovered a broad spectrum of Cort-responsive genes, potentially involved in the regulation of the HPA-axis, as well as other neuroendocrine and pre-autonomic functions of the PVH.

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Glucocorticoid-Dependent Hippocampal Transcriptome in Male Rats: Pathway-Specific Alterations with Aging

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