Triflusal reduces dense-core plaque load, associated axonal alterations and inflammatory changes, and rescues cognition in a transgenic mouse model of Alzheimer's disease

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Abstract

Inflammation has been associated with the two classic lesions in the Alzheimer's (AD) brain, amyloid deposits and neurofibrillary tangles. Recent data suggest that Triflusal, a compound with potent anti-inflammatory effects in the central nervous system in vivo, might delay the conversion from amnestic mild cognitive impairment to a fully established clinical picture of dementia. In the present study, we investigated the effect of Triflusal on brain Aβ accumulation, neuroinflammation, axonal curvature and cognition in an AD transgenic mouse model (Tg2576). Triflusal treatment did not alter the total brain Aβ accumulation but significantly reduced dense-cored plaque load and associated glial cell proliferation, proinflammatory cytokine levels and abnormal axonal curvature, and rescued cognitive deficits in Tg2576 mice. Behavioral benefit was found to involve increased expression of c-fos and BDNF, two of the genes regulated by CREB, as part of the signal transduction cascade underlying the molecular basis of long-term potentiation. These results add preclinical evidence of a potentially beneficial effect of Triflusal in AD.

Introduction

Neuropathological and epidemiological data suggest that inflammation plays an important role in Alzheimer's disease (AD) (Wyss-Coray, 2006). Abundant activated microglia and astrocytes, and release of proinflammatory mediators have been invariantly identified in AD brains in association with the classic extracellular deposits of amyloid called senile plaques (SP), in particular with dense-core or fibrillar Aβ-containing plaques, and are believed to contribute to neurotoxicity (Akiyama et al., 2000, Koenigsknecht and Landreth, 2004). In addition, multiple epidemiological investigations point to a risk reduction in long-term users of non-steroidal anti-inflammatory drugs (NSAIDs) (McGeer and McGeer, 1996, Wyss-Coray, 2006). The biological mechanism underlying this protection is not completely understood. Even though the main activity of NSAIDs is to inhibit cyclooxygenase (COX) activity, other mechanisms of action have been proposed for activity of certain NSAIDs such inhibition of amyloid-β formation, PPAR-gamma agonism and NFkappaB negative regulation or stimulation of neurotrophin synthesis, that could be potentially beneficial in AD (reviewed in Imbimbo, 2009). Multiple studies have tested the effects of traditional NSAIDs on transgenic mouse models of AD. Consistent in most of these studies there is a reduction of activated microglia in many cases accompanied by a decrease in Aβ load (reviewed in McGeer and McGeer, 2007). However, placebo-controlled clinical trials with NSAIDs in AD patients have produced negative results so far (reviewed in McGeer and McGeer, 2007). A trial with rofecoxib in individuals with mild cognitive impairment (MCI) was also negative (Thal, 2005). The primary prevention study with naproxen and celecoxib (ADAPT) in elderly subjects at high risk of dementia was terminated early because of cardiovascular risks associated with both these agents, and meaningful data on the study's primary outcome (prevention of AD) were not available (Martin, 2008). Of note, most of the above clinical trials have involved agents with a selective action against COX-2. However there are no epidemiological data indicating that COX-2 selective inhibitors would decrease the risk of AD and, as opposed to traditional NSAIDs that inhibit COX-1 with variable actions against COX-2, they conferred no protection or even increased the levels of Aβ, like in the case of celecoxib, when tested in AD transgenic mouse models (Kukar et al., 2005).

We have recently conducted a randomized placebo-controlled trial of Triflusal in patients with amnestic MCI (aMCI), in many cases the earliest clinical phases of AD, and found that Triflusal significantly lowered the rate of conversion from aMCI to dementia of AD type (Gomez-Isla et al., 2008). Triflusal (2-acetoxy-4-trifluoromethylbenzoic acid), a widely used platelet antiaggregant with structural similarities to salicylates platelet but which is not derived from aspirin, is a COX-1 inhibitor that also inhibits to a lesser extent inflammatory induced COX-2 expression (reviewed in Murdoch and Plosker, 2006). After oral administration, Triflusal is rapidly hydrolysed to its active metabolite 2-hydroxy-4-trifluoromethyl-benzoic acid (HTB), which is able to cross the blood brain barrier as it has been recently shown in healthy volunteers (Valle et al., 2005). Triflusal is well tolerated and has a reduced risk of haemorrhagic complications in comparison to aspirin (reviewed in Murdoch and Plosker, 2006). In vitro studies have demonstrated that Triflusal and HTB are potent inhibitors of NFkappaB activation and the subsequent expression of several cytokines and COX-2 (Fernández de Arriba et al., 1999, Bayon et al., 1999, Hernández et al., 2001). Moreover orally administered Triflusal conferred robust neuroprotection and downregulated glial activation and expression of NFkappaB-regulated proinflammatory genes, like IL-1β, TNF-α or COX2, in postnatal rat models of cerebral ischemia and excitotoxicity (Acarin et al., 2001, Acarin et al., 2002).

In the present study we hypothesized that Triflusal may also help protect the brain against AD-related neuroinflammatory mechanisms and improve cognition in vivo. To test this hypothesis, Tg2576 mice overexpressing human mutant APP (Swedish mutation KM670/671NL) (Hsiao et al., 1996) received Triflusal or vehicle beginning at 10 months of age for 3 consecutive months and at 13 months of age for 5 consecutive months. Triflusal treatment significantly reduced compact (fibrillar, Thioflavin-S reactive) plaque load and associated neuroinflammatory changes (activated glial cells and levels of proinflammatory IL-1β and TNF-α), and significantly improved altered neurite trajectories both near and far away from cored plaques in the brain of Tg2576 mice. Moreover, Triflusal treatment rescued spatial learning and context fear conditioning deficits, and up-regulated expression of c-fos and BDNF, two of the genes regulated by CREB (cyclic AMP response element binding protein) as part of the signal transduction cascade underlying the molecular mechanisms of neuronal plasticity. These results further suggest that Triflusal may be therapeutically beneficial in AD.

Section snippets

Animals

Tg2576 mice overexpressing human mutant APP (Swedish mutation KM670/671NL) on a mixed hybrid genetic background C57Bl6j/SJL were used in this study (Hsiao et al., 1996).

Groups of Tg2576 mice (Tg+) were administered Triflusal (n = 19) or vehicle (n = 20) starting at 10 months of age during 3 consecutive months and used for subsequent clinicopathological and biochemical analyses. A group of age and gender-matched non-transgenic littermate controls (Tg−) received vehicle (n = 20) on a similar timetable

Results

The average plasma concentration of Triflusal active metabolite HTB determined in mice killed 24 h after final administration was 47.13±24.70 µg/ml.

Discussion

Inflammatory changes, including the presence of abundant activated microglia and astrocytes, and release of proinflammatory mediators are consistently identified in association with the classic “neuritic” or dense-core fibrillar Aβ-containing plaques from early stages of the disease process in AD brains and transgenic mouse models, suggesting that inflammation is an important player in the pathogenesis of the disease. Of note, this subpopulation of “neuritic” plaques surrounded by clusters of

Acknowledgments

This project was funded in part by FIS PI041887, CIBERNED, NIH AG020570 and Marato 072610.

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    1

    These authors equally contributed to this work.

    2

    Current address: Neurology Department, Massachusetts General Hospital, WACC Suite 715, 15th Parkman St, Boston MA 0214, USA. Fax: +1 617 726 4101.

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