Immune response gene expression increases in the aging murine hippocampus

Abstract

Using GeneChips®, basal and lipopolysaccharide (LPS)-induced gene expression was examined in the hippocampus of 3-, 12-, 18- and 24-month-old male C57BL/6 mice to identify genes whose altered expression could influence hippocampal function in advanced age. Gene elements that changed with age were selected with a t-statistic and specific expression patterns were confirmed with real-time quantitative PCR. Basal expression of 128 gene elements clearly changed with age in the hippocampus. Fourteen gene elements showed increased expression with age and these increases were validated after LPS stimulation. Major histocompatibility complex (MHC) TL region and thymic shared antigen (TSA-1) gene expression increased, suggesting T cell activation in the hippocampus with age. Cytokine (interleukin (IL)-1β, tumor necrosis factor (TNF)-α) and chemokine (macrophage chemotactic protein-1) expression increased sharply in 24-month-old mice. These findings are in contrast to a decrease in the peripheral immune response, documented by decreased T cell proliferation and decreased ratios of naive to memory T cells. Age-related increases in inflammatory potential in the brain may contribute to neurodegenerative diseases of the aged.

Introduction

A frequent correlate of human aging is a decline in learning, cognition and memory. Age-related functional decreases in short-term and spatial memory and declines in memory performance have been confirmed in rodent and primate models Moss et al., 1997, Herndon et al., 1997, Gage et al., 1984, Ward et al., 1999. These changes can be attributed to alterations in functions of hippocampal pathways (Lanahan et al., 1997). In humans, the extreme is Alzheimer's disease (AD), a progressive and irreversible loss of memory and cognition, as a result of amyloid plaques in the neurons of the hippocampus and cerebral cortex. Immune response genes are expressed in the hippocampus and have been demonstrated to affect long-term potentiation (LTP) and to be associated with the inflammatory response to amyloid beta in AD. An investigation of age-related changes in expression of hippocampal genes is the first step in determining whether immune response gene expression plays a role in the alterations in learning and memory that occur in old age.

Although immunological function in the periphery has been demonstrated to decline with age (Miller, 1996), cells with immunological capability in the brain become more activated with age. Age-related increases in activation of astrocytes, characterized by increased glial fibrillary acidic protein (GFAP) expression Goss et al., 1991, Morgan et al., 1997, Nichols et al., 1993, O'Callaghan and Miller, 1991, and microglia have been reported in various species Mattiace et al., 1990, Sheng et al., 1998, Peters et al., 1991, Sheffield and Berman, 1998, Ogura et al., 1994, Rozovsky et al., 1998. Microglial activation is manifested by increased phagocytic activity (Dickson et al., 1990), increased expression of major histocompatibility complex (MHC) class II genes Mattiace et al., 1990, Sheffield and Berman, 1998 and increased secretion of proinflammatory cytokines Sheng et al., 1998, Nanamiya et al., 2000, Takao et al., 1996. Immune response genes, in particular those coding cytokines and their receptors, are expressed in high density in the hippocampus Besedovsky and del Rey, 1996, Haas and Schauenstein, 1997, Murray et al., 1999, Rothwell and Hopkins, 1995 and interleukin (IL)-1, as well as IL-6 and tumor necrosis factor (TNF)-α, affect the induction of hippocampal LTP. IL-1β, IL-6 and TNF-α have all been reported to be involved in the formation of neuritic plaques in AD Griffin et al., 1995, Patterson, 1995, Strauss et al., 1992 and anti-inflammatory agents have been shown to slow the progression of AD (Breitner et al., 1995).

In this study, we have examined changes in gene expression that occur in the hippocampus during the aging process using the GeneChip® technology. Previous gene chip studies on aging have examined gene expression in the cortex and cerebellum (Lee et al., 2000). Microarray studies of the rodent hippocampus have focused on subregional expression within this structure (Zhao et al., 2001) and the effect of exercise on gene expression (Tong et al., 2001). A recent study has compared a small number of genes in the hippocampus of young (2 months) and old (20 months) rats (Cho et al., 2002). In the present study, we examined hippocampal mRNA from young (3 months), middle-aged (12 and 18 months) and old (24 months) male C57BL/6 mice. A range of ages was included to determine whether changes in gene expression that were observed occurred gradually or appeared abruptly at a specific time during the aging process. The results obtained in the basal aging experiments were further validated by intraperitoneal (i.p.) LPS administration at each of the four ages, a procedure known to increase expression of cytokines, including IL-1β in the hippocampus Dantzer et al., 1998, Laye et al., 1994.