Research ReportHippocampal gene expression changes during age-related cognitive decline
Introduction
Brain aging leading to cognitive decline is a process that is not well understood. The hippocampus is integral to memory function and is a brain area greatly affected by aging. Some individuals have a greater loss of neuronal plasticity than others, leading to an accelerated deterioration of memory function as they age.
Numerous studies in rats (Blalock et al., 2003, Burger et al., 2007, Rowe et al., 2007, Burger et al., 2008) and mice (Verbitsky et al. 2004) have documented age-related memory impairment using the Morris water maze (MWM). Using microarrays, these studies have found significant changes in genes related to cholesterol synthesis, inflammation, transcription factors, neurogenesis and synaptic plasticity. A decline in memory performance utilizing the MWM from young to middle-aged to aged rats was observed by Blalock et al. with only changes between the young and aged rats reaching statistical significance. The gene expression profiling of the CA1 region of the hippocampus of these animals found genes involved in cell signaling, metabolism, transcription factors and neurite plasticity to be down-regulated in aged rats. Up-regulated in the aging animals were inflammation, growth, glial, signal transduction and protein vesicle genes. A similar study (Verbitsky et al. 2004) comparing young and middle-aged mice found a decline of memory function associated with aging based on MWM performance. The hippocampal gene expression results of this study found genes associated with inflammation, iron homeostasis, calcium dependent phospholipid binding and acetyl-CoA biosynthesis to be up-regulated in the middle aged animals.
Other studies have observed that some aged rats have a decline in spatial memory as measured by the MWM, while others perform as well as young animals. Microarray analysis of the hippocampal gene expression revealed significant differences between young, AU and AI animals that represent a variety of functions (Burger et al., 2007, Rowe et al., 2007, Burger et al., 2008). The study by Rowe found more genes associated with aging were differentially expressed than genes just associated with impairment. Both Burger and Rowe found that genes associated with neuron growth and cell morphology were down-regulated in aging animals.
Terao et al. (2002) examined the changes in hippocampal gene expression in 3, 12, 18 and 24 month old mice, both with and without immune system stimulation with lipopolysaccharide (LPS). They discovered that 128 genes were significantly differentially expressed with aging and 14 with immune system stimulation and aging. Eight of these genes overlapped, and all had immune system or antioxidant functions. This study found that most hippocampal gene expression age-related changes were increases in expression and a great number of these transcripts were associated with immune response.
In this study we have also observed gene expression changes in the hippocampus of animals who demonstrated memory impairment on the MWM when compared to their age-matched peers. This comparison was made by observing that some aged (24 months) animals performed as well as young (3 months) animals, while another group of aged animals were memory impaired compared to young animals. Microarray analysis revealed very few changes between young and aged-unimpaired (AU) animals, but found significant differences between memory unimpaired animals and aged-impaired animals (AI). We have the advantage of utilizing the most recent mouse expression arrays (Affymetrix 430) with more than 30,000 transcripts to allow us to further investigate subtle gene expression changes that may influence memory impairment. By using a robust statistical test, we were able to concentrate on a narrow list of 19 differentially expressed genes. This focused list allowed us to do a detailed pathway analysis that uncovered the role of heat shock 70 kDa protein 5 (Hspa5, also known as BiP or GRP78) and calreticulin (Calr) in the regulation of apolipoprotein B (Apob) and also the up-regulation of myelin oligodendrocyte glycoprotein (Mog). Mog has been observed to have demyelinating and inflammatory effects. We hypothesize that this combination of higher Apob, allowed by the down-regulation of Hspa5 and Calr, and higher expression of Mog causes inflammation and lipid changes that cause a subtle degradation in the hippocampus leading to memory impairment.
Section snippets
Maze learning
Mean swim times for the 10 spatial learning blocks are shown in Fig. 1. All groups showed improvement over the blocks. However, two-way ANOVA indicated a significant interaction between group and training block (F = 1.72, p = 0.03, as well as significant main effects of Group (F = 8.45, p = 0.0009) and training block (F = 2.94, p = 0.0001). Follow-up ANOVAs compared each of the aged groups to the young group individually. When AU were compared with the young group, there was not a significant interaction (F
Discussion
This study adds to previous works that have discussed age-related spatial memory deficits in rats and mice (Blalock et al., 2003, Verbitsky et al., 2004, Burger et al., 2007, Burger et al., 2008). Unique to our study is the utilization of a 39,000 probe chip and an age-matched group consisting of 24 month animals who demonstrated spatial memory performance on par with young mice versus 24 month mice with a clearly impaired spatial memory performance. The memory performance of AU animals
Animals
Male C57BL/6J Nia mice aged 23 and 2 months old were obtained from Charles River Laboratories. Behavioral assessments were begun 1 month after the arrival of the mice. They were housed individually in clear polypropylene cages in a temperature- and humidity-controlled room with an artificial 12 h:12 h light:dark cycle, lights on at 0700 h. Standard pelleted chow and water were freely available at all times.
Behavioral testing
Learning was conducted in a water maze, which consisted of a galvanized tank 122 cm in
Acknowledgments
The authors wish to recognize the support of the State of Arizona, Science Foundation Arizona, and the Arizona Alzheimer's Consortium.
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