Chronic anti-murine Aβ immunization preserves odor guided behaviors in an Alzheimer's β-amyloidosis model

Abstract

Olfaction is often impaired in Alzheimer's disease (AD) and is also dysfunctional in mouse models of the disease. We recently demonstrated that short-term passive anti-murine-Aβ immunization can rescue olfactory behavior in the Tg2576 mouse model overexpressing a human mutation of the amyloid precursor protein (APP) after β-amyloid deposition. Here we tested the ability to preserve normal olfactory behaviors by means of long-term passive anti-murine-Aβ immunization. Seven-month-old Tg2576 and non-transgenic littermate (NTg) mice were IP-injected biweekly with the m3.2 murine-Aβ-specific antibody until 16 mo of age when mice were tested in the odor habituation test. While Tg2576 mice treated with a control antibody showed elevations in odor investigation times and impaired odor habituation compared to NTg, olfactory behavior was preserved to NTg levels in m3.2-immunized Tg2576 mice. Immunized Tg2576 mice had significantly less β-amyloid immunolabeling in the olfactory bulb and entorhinal cortex, yet showed elevations in Thioflavin-S labeled plaques in the piriform cortex. No detectable changes in APP metabolite levels other than Aβ were found following m3.2 immunization. These results demonstrate efficacy of chronic, long-term anti-murine-Aβ m3.2 immunization in preserving normal odor-guided behaviors in a human APP Tg model. Further, these results provide mechanistic insights into olfactory dysfunction as a biomarker for AD by yielding evidence that focal reductions of Aβ may be sufficient to preserve olfaction.

Introduction

Olfactory perceptual impairments are commonly reported in Alzheimer's disease (AD). In particular, persons with AD often display reduced abilities to detect, discriminate, and identify odors (for review [1], [2]). These impairments in olfaction are even reported to precede significant cognitive dysfunction [3], highlighting the vulnerability of the olfactory system to the early events of AD and the possible clinical utility of olfactory dysfunction as a biomarker for the disease (e.g., [4], [5]). Understanding the mechanisms of olfactory perceptual loss in AD may help to elucidate general principles of disease pathogenesis and will be critical in treating olfactory dysfunction in the disease.

Olfactory perception requires that odor information originating with the binding of odorants to olfactory receptor neurons in the nose be transferred throughout multiple brain regions essential to odor processing. Following the initial events of odor processing within the olfactory bulb (OB) [6], odor information travels into olfactory cortices, including the piriform cortex (PCX) wherein processes critical for odor habituation and olfactory learning occur [7], [8], [9], [10], [11], [12]. Odor information then enters the lateral entorhinal cortex (EC) [13], [14], [15] and ultimately the hippocampus (hipp) for odor memory storage and future retrieval [16]. The normal function of this network, which is well conserved through evolution and highly similar in rodent and human [17], is critical for olfactory perception, and indeed disrupting odor information flow throughout any of these regions can impair olfactory perception (e.g., [15], [18], [19], [20], [21], [22]).

While the neural basis for olfactory impairments in AD remain unclear, recent work from AD mouse models has suggested a role for amyloid-β (Aβ) in disrupting normal olfactory network function and olfactory behaviors [23], [24], [25], [26]. Recent work from our group [26] in the Tg2576 mouse overexpressing human APP with the Swedish familial AD mutation demonstrated that behavioral dysfunction in the odor habituation task positively correlates with levels of fibrillar and non-fibrillar Aβ within olfactory structures, including the OB, PCX, EC, and hipp. Indeed, dysfunction in various olfactory behaviors has been reported in multiple AD model mouse lines [24], [27], [28], [29], [30]. More recently, we reported that OB and PCX neural activity is highly aberrant in Tg2576 transgenic mice and that this is restored to near wild type levels following acute pharmacological intervention to lower Aβ levels [23], [25]. Thus, it is likely that Aβ and/or other factors related to APP processing are responsible for decline in olfactory system function. Exploring anti-Aβ strategies as potential therapies against olfactory perturbations in this model may provide insights into mechanisms of sensory decline in AD and its treatment.

We recently demonstrated that acute (short-term) passive anti-murine-Aβ immunization can rescue olfactory behavioral impairments in the Tg2576 mouse model [31]. In this study, 8 week treatment with the anti-murine Aβ antibody, m3.2, which is a monoclonal antibody with a selective affinity for murine Aβ (mAβ) [32], was found to have reduced both brain mAβ and human Aβ (hAβ) levels and also preserved normal odor habituation behaviors in Tg2576 mice when the immunization was begun after significant β-amyloid deposition. As summarized in Table 1, this 8 week treatment study showed that acute (short-term) passive anti-murine-Aβ immunization lowered brain Aβ levels in aged Tg2576 mice without altering other measured APP metabolite levels. However, whether these behavioral changes are accompanied by altered Aβ burden specifically in the olfactory system and whether similar findings can be observed following chronic treatment beginning at the earliest stages of Aβ deposition remain to be determined. Therefore, here we tested the hypothesis that ongoing anti-mAβ immunization would prevent the deposition of Aβ within the brain, specifically within olfactory structures, and thereby within those very same animals, preserve normal odor habituation behavior.