Extensive innate immune gene activation accompanies brain aging, increasing vulnerability to cognitive decline and neurodegeneration: a microarray study

Abstract

Background: This study undertakes a systematic and comprehensive analysis of brain gene expression profiles of immune/inflammation-related genes in aging and Alzheimer's disease (AD).

Methods: In a well-powered microarray study of young (20 to 59 years), aged (60 to 99 years), and AD (74 to 95 years) cases, gene responses were assessed in the hippocampus, entorhinal cortex, superior frontal gyrus, and post-central gyrus.

Results: Several novel concepts emerge. First, immune/inflammation-related genes showed major changes in gene expression over the course of cognitively normal aging, with the extent of gene response far greater in aging than in AD. Of the 759 immune-related probesets interrogated on the microarray, approximately 40% were significantly altered in the SFG, PCG and HC with increasing age, with the majority upregulated (64 to 86%). In contrast, far fewer immune/inflammation genes were significantly changed in the transition to AD (approximately 6% of immune-related probesets), with gene responses primarily restricted to the SFG and HC. Second, relatively few significant changes in immune/inflammation genes were detected in the EC either in aging or AD, although many genes in the EC showed similar trends in responses as in the other brain regions. Third, immune/inflammation genes undergo gender-specific patterns of response in aging and AD, with the most pronounced differences emerging in aging. Finally, there was widespread upregulation of genes reflecting activation of microglia and perivascular macrophages in the aging brain, coupled with a downregulation of select factors (TOLLIP, fractalkine) that when present curtail microglial/macrophage activation. Notably, essentially all pathways of the innate immune system were upregulated in aging, including numerous complement components, genes involved in toll-like receptor signaling and inflammasome signaling, as well as genes coding for immunoglobulin (Fc) receptors and human leukocyte antigens I and II.

Conclusions: Unexpectedly, the extent of innate immune gene upregulation in AD was modest relative to the robust response apparent in the aged brain, consistent with the emerging idea of a critical involvement of inflammation in the earliest stages, perhaps even in the preclinical stage, of AD. Ultimately, our data suggest that an important strategy to maintain cognitive health and resilience involves reducing chronic innate immune activation that should be initiated in late midlife.

Figure 1

Immune-related genes undergo more extensive response in the course of cognitively normal aging (age 20 to 99) than in Alzheimer’s disease (AD). (A) In aging, comparing gene expression levels in young (20 to 59 yrs) versus aged (60 to 99 yrs) individuals revealed that numerous immune-related gene changes occur in the superior frontal gyrus (SFG), post-central gyrus (PCG) and hippocampus (HC), with fewer genes showing significant change in the entorhinal cortex (EC) with age. The majority of gene responses were increased expression with age, in all brain regions assessed. (B) Relative to aging, fewer immune genes showed significant change in AD versus age-matched controls, with gene responses primarily restricted to the HC and SFG. Negligible immune gene expression change was observed in the EC and PCG in AD. (C) A subset of immune-related genes underwent progressive change across aging and AD, particularly in the HC and SFG, predominantly undergoing increased expression across aging and AD. Few immune genes underwent progressive change across aging and AD in the EC and PCG.

Figure 2

Immune-related genes undergo gender and region-specific patterns of response in aging and Alzheimer’s disease (AD). In both females and males, the extent of immune-gene response is greater in aging (A, D) than in AD (B, E), with gender-specific patterns of response across brain regions. In aging, females show the greatest number of genes responding in the hippocampus (HC) (A), while the most responsive region in males was the superior frontal gyrus (SFG) (D). In both genders, the entorhinal cortex (EC) showed the fewest numbers of responding genes over the course of aging, the post-central gyrus (PCG) underwent an intermediate response, and the direction of gene change was predominantly upregulated in all brain regions. In AD (B, E), males and females both showed a limited number of significant gene responses, with a greater number of significant gene changes observed in females (B) relative to males (E), particularly in the EC and HC. For genes undergoing progressive change across aging and AD (C, F), few such genes were apparent in females (C), with a relatively large number genes following this pattern in the SFG in males, with the majority of these genes undergoing progressively increased expression across age and AD

Figure 3

qPCR was used to assess expression profiles of several genes for which gene expression was below microarray detection sensitivity. qPCR analysis of hippocampal gene expression for the proinflammatory cytokines IL-6, IL-1beta, NF-alpha, and the anti-inflammatory cytokine IL-10 revealed significant upregulation of IL-6, IL-1beta and IL-10 with age, and further upregulation of IL-10 in Alzheimer’s disease (AD) (A). Similarly, modulators of cytokine signaling were upregulated in the hippocampus (HC) with age including a six-fold increase in the suppressor of cytokine signaling (SOCS-3) and a two-fold increase in IRAK3, a serine/threonine kinase that mediates signaling from toll-like receptors (TLRs) and IL-1 receptor family members (B). In contrast, no significant change in components of the inflammasome (NRLP3 and ASC) was detected in the superior frontal gyrus (SFG) with aging or AD (C). **** P < 0.0001, ***P < 0.005, *P < 0.05.

Figure 4

Gene expression profiles in young, aged, and Alzheimer’s disease (AD) samples using hippocampal tissue show similar patterns of change with qPCR and microarray analysis. qPCR (A,B) demonstrated significant gene upregulation of CD14, TLR2, TL4, TLR7, MYD88 and downregulation of TOLLIP in aging, confirming microarray results (C,D). **** P < 0.0001, ***P < 0.005, **P < 0.01, *P < 0.05.