Elsevier

Neurobiology of Aging

Volume 34, Issue 4, April 2013, Pages 1051-1059
Neurobiology of Aging

Regular article
Neuroinflammatory phenotype in early Alzheimer's disease

https://doi.org/10.1016/j.neurobiolaging.2012.09.012Get rights and content

Abstract

Alzheimer's disease (AD) involves progressive neurodegeneration in the presence of misfolded proteins and poorly-understood inflammatory changes. However, research has shown that AD is genetically, clinically, and pathologically heterogeneous. In frozen brain samples of frontal cortex (diseased) and cerebellum (nondiseased) from the University of Kentucky Alzheimer's Disease Center autopsy cohort, we performed gene expression analysis for genes categorizing inflammatory states (termed M1 and M2) from early and late stage AD, and age-matched nondemented controls. We performed analysis of the serum samples for a profile of inflammatory proteins and examined the neuropathologic data on these samples. Striking heterogeneity was found in early AD. Specifically, early-stage AD brain samples indicated apparent polarization toward either the M1 or M2 brain inflammatory states when compared with age-matched nondisease control tissue. This polarization was observed in the frontal cortex and not in cerebellar tissue. We were able to detect differences in AD neuropathology, and changes in serum proteins that distinguished the individuals with apparent M1 versus M2 brain inflammatory polarization.

Introduction

Alzheimer's disease (AD) is a neurodegenerative disorder characterized clinically by a progressive loss of cognitive function. Pathologically, AD is defined by the presence of amyloid plaques composed of extracellular aggregated amyloid-β (Aβ) protein, and neurofibrillary tangles comprised of intracellular aggregated, hyperphosphorylated tau protein (Hyman, et al., 2012). Accompanying the primary pathologies of AD is an inflammatory response, mediated partly by the microglial cells in the brain but also involving astrocytes, oligodendrocytes, pericytes, and neurons (see review, Colton and Wilcock, 2010). The upregulation of inflammatory signaling in the brain, termed neuroinflammation, has been shown to occur in all cases of AD to varying degrees (reviewed in Akiyama, et al., 2000).

Studies in transgenic mouse models have revealed a diverse role for neuroinflammation in the progression of amyloid plaques and neurofibrillary tangles. Intracranial injection of lipopolysaccharide (LPS), a classic stimulator of inflammation, induces neuroinflammation and results in amyloid plaque reduction in APP+PS1 transgenic mice (DiCarlo, et al., 2001). Additionally, anti-Aβ antibodies injected intracranially or administered systemically in APP transgenic mice results in a neuroinflammatory response and subsequent reductions in amyloid plaques (Wilcock, et al., 2003, 2004). In contrast to the effects of LPS on amyloid plaque injection of LPS into the brains of rTg4510 tau mice results in exacerbation of tau pathology (Lee, et al., 2010) suggesting that tau pathology and amyloid pathology respond differentially to neuroinflammatory stimuli.

Recently, Colton et al. showed that inflammatory markers expressed in the brains of amyloid-depositing mice and human AD tissue are not restricted to classic inflammatory genes such as interleukin (IL)-1β or tumor necrosis factor (TNF)α; additional inflammatory markers such as arginase-1, chitinase-3-like proteins, and mannose receptor are also highly expressed (Colton, et al., 2006). In contrast to the cytotoxic effects that occur with agents such as IL-1β and TNFα, the additional inflammatory genes are known to be essential for wound healing and repair and might therefore reflect attempts at brain repair in response to the presence of AD pathologies.

We hypothesized that the characteristics of neuroinflammatory signaling in the early stages of clinical AD would associate with clinical and pathological parameters in a biologically informative way. To address this, we performed gene expression analysis for a profile of neuroinflammatory genes in frozen tissue samples from patients clinically classified as mild AD or late AD and compared these with age-matched nondemented controls. We examined the frontal cortex as our diseased brain region and cerebellum as our nondiseased region. Microglia, the central orchestrators of central nervous system inflammation, are derived from the same lineage as macrophages so we have chosen to use the same nomenclature for the neuroinflammatory phenotypes as is used in phenotyping macrophages: M1 and M2 (reviewed in Mantovani, et al., 2004 and Mosser and Edwards, 2008). M1 macrophages are associated with expression of classic inflammatory cytokines such as IL-1β and TNFα and M2 macrophages are associated with expression of wound repair and healing mediators such as arginase-1, chitinase-like proteins, and anti-inflammatory cytokines IL-4 and IL-10 (Edwards, et al., 2006; Martinez, et al., 2009; Mosser, 2003).

Section snippets

Tissue samples

All tissue samples were obtained from the University of Kentucky Alzheimer's Disease Center at the Sanders-Brown Center on Aging. Details of the University of Kentucky Alzheimer's Disease Center recruitment have been described previously (Nelson, et al., 2007; Schmitt, et al., 2001, 2012). A series of cognitive tests were performed including the Mini Mental State Examination (MMSE; Folstein, et al., 1975). Pathologic assessments were performed at the University of Kentucky as described

Results

The neuroinflammatory genes analyzed are shown in Fig. 1A. When we examined the data from the frontal cortex of our early AD tissue we found some highly variable gene expression. We performed K-means cluster analysis on the expression data from all 23 frontal cortex samples to determine whether there were patterns of gene expression. As can be seen in Fig. 1B, our samples separated into 2 distinct clusters. Cluster 1 showed very high expression levels of FIZZ, IL-1Ra, MRC1, and AG1. Cluster 2

Discussion

We hypothesized that neuroinflammatory gene expression would provide biologically informative insights into the heterogeneous clinical and pathologic context of the AD brain. We chose to categorize our neuroinflammatory genes based on the macrophage phenotypes of M1, M2a, M2b, and M2c (reviewed in Mosser and Edwards, 2008). We obtained frozen frontal cortex as our diseased brain region and frozen cerebellum as our nondiseased brain region from early AD samples, late AD samples, and age-matched

Disclosure statement

The authors have no conflicts of interest to disclose.

Studies conformed to the institutional review board protocols regarding the use of human subjects in research.

Acknowledgements

The project described was supported in part by NIH grant P30 AG028383 and by the National Center for Research Resources and the National Center for Advancing Translational Sciences, National Institutes of Health, through Grant UL1RR033173. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH. The authors thank all of the study volunteers, and Sonya Anderson, Ela Patel, and Erin Abner for technical support, Greg Cooper, MD, Greg

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