In vivo and in vitro evidence supporting a role for the inflammatory cytokine interleukin-1 as a driving force in Alzheimer pathogenesis

Abstract

Interleukin-1 (IL-1), an inflammatory cytokine overexpressed in the neuritic plaques of Alzheimer's disease, activates astrocytes and enhances production and processing of beta-amyloid precursor protein (beta-APP). Activated astrocytes, overexpressing S100 beta, are a prominent feature of these neuritic plaques, and the neurite growth-promoting properties of S100 beta have been implicated in the formation of dystrophic neurites overexpressing beta-APP in neuritic plaques. These facts collectively suggest that elevated levels of the inflammatory cytokine IL-1 drive S100 beta and beta-APP overexpression and dystrophic neurite formation in Alzheimer's disease. To more directly assess this driver potential for IL-1, we analyzed IL-1 induction of S100 beta expression in vivo and in vitro, and of beta-APP expression in vivo. Synthetic IL-1 beta was injected into the right cerebral hemispheres of 13 rats. Nine additional rats were injected with phosphate-buffered saline, and seven rats served as uninjected controls. The number of astrocytes expressing detectable levels of S100 beta in tissue sections from IL-1-injected brains was 1.5 fold that of either control group (p < 0.01), while tissue S100 beta levels were approximately threefold that of controls (p < 0.05). The tissue levels of two beta-APP isoforms (approximately 130 and 135 kDa) were also significantly elevated in IL-1-injected brains (p < 0.05). C6 glioma cells, treated in vitro for 24 h with either IL-1 beta or IL-1 alpha, showed significant increases in both S100 beta and S100 beta mRNA levels. These results provide evidence that IL-1 upregulates both S100 beta and beta-APP expression, in vivo and vitro, and support the idea that overexpression of IL-1 in Alzheimer's disease drives astrocytic overexpression of S100 beta, favoring the growth of dystrophic neurites necessary for evolution of diffuse amyloid deposits into neuritic beta-amyloid plaques.

FIG. 1

S100β-immunoreactivity (S100β⁺) in tissue sections of rat cerebrum showing increases in number, size, and immunoreactive intensity of S100β⁺ astrocytes (brown) in IL-1β–injected animals. As illustrated here, some S100β⁺ astrocytes could also be found in PBS-injected animals, but the number of these cells was not significantly different from controls (see Fig. 2). (A) Uninjected control rat; (B) PBS-injected rat; (C) IL-1β–injected rat. Bars = 15 μm.

FIG. 2

Numbers of S100β-immunoreactive cells in five separate 250-diameter fields in brains from five IL-1β–injected; three PBS-injected, and three uninjected control rats.

FIG. 3

Examples of Western immunoblot showing increased expression of S100β in tissue extracts from IL-1β–injected (IL-1β–injection) brain compared to PBS-injected (PBS injection) brain. Results from three IL-1β–injected and three PBS-injected animals are shown.

FIG. 4

IL-1 upregulation of S100β and S100β mRNA in C6 glioma cells in vitro. (A) Northern hybridization blots showing S100β mRNA with corresponding 28S and 18S levels from IL-1β–treated (β), IL-1α–treated (α), and untreated (C) cells. (B) Western immunoblot probed with anti-S100β and anti-GFAP antibodies showing results from IL-1β–treated (β), IL-1α–treated (α), and untreated (C) cells. (C) S100β and S100β mRNA levels in IL-1β–treated, IL-1α–treated, and untreated (control) C6 glioma cells. *p = 0.02; **p < 0.002 compared to control values.

FIG. 5

S100β mRNA levels (radioactive counts per min, CPM) in IL-1β–treated C6 glioma cells analyzed using micro in situ hybridization. There is a significant increase in S100β mRNA following 24 h of IL-1β treatment (*p < 0.05).

FIG. 6

Example of Western immunoblot of two β-APP–immunoreactive bands, at 130 kDa and 135 kDa, showing increased expression of both β-APP isoforms in IL-1β–injected brain (IL-1β injection) compared to PBS-injected (PBS-injection) brain. Results from three IL-1β–injected and three PBS-injected animals are shown.