Elsevier

Neurobiology of Aging

Volume 62, February 2018, Pages 20-33
Neurobiology of Aging

Regular article
The mechanisms underlying olfactory deficits in apolipoprotein E–deficient mice: focus on olfactory epithelium and olfactory bulb

https://doi.org/10.1016/j.neurobiolaging.2017.09.036Get rights and content

Abstract

Apolipoprotein E (ApoE) is highly expressed in the central nervous system including the olfactory epithelium (OE) and olfactory bulb (OB). ApoE induction is beneficial for Alzheimer's disease (AD) treatment, whereas ApoE deficiency results in impaired olfaction, but the timing and underlying molecular and cellular mechanisms of these effects remain unclear. Uncovering the mechanisms underlying olfactory dysfunction in ApoE-deficient mice might provide a potential avenue for the early diagnosis of AD. We used an ApoE knockout (ApoE−/−) mouse model and a cookie-finding test to reveal an olfactory deficit in 3- to 5-month-old, but not 1- to 2-month-old, ApoE−/− mice. Electrophysiological experiments indicated a significant decline in the electroolfactogram (EOG) amplitude, which was associated with an increase in rise time in ApoE−/− mice. Knockout mice also exhibited compromised olfactory adaptation, as well as a reduced number of mature olfactory sensory neurons in the OE. Local field potential recording in the OB showed that gamma oscillation power was enhanced, which might be attributed to an increase in GABAergic inhibition mediated by parvalbumin-expressing (PV) interneurons. This study demonstrates the critical involvement of ApoE in olfactory information processing in the OE and OB. ApoE deficiency results in olfaction deficits in mice as young as 3 months old, which has implications for AD pathogenesis.

Introduction

Alzheimer's disease (AD) is a progressive neurodegenerative disease wherein patients suffer from sensory, motor, progressive memory loss, and cognitive decline, which accounts for 60%–70% of all cases of dementia (Daulatzai, 2010). AD patients often have neuropathological changes in components of the brain involved in olfactory processing, along with a reduced ability to detect, discriminate, and identify odors (Ferrer et al., 2016, Murphy, 1999). In fact, olfactory dysfunction often precedes other clinical symptoms in chronic neurodegenerative diseases (Djordjevic et al., 2008, Zhang, 2016) and has a relatively high prevalence in various types of dementia, occurring in up to 100% of AD patients (Duff et al., 2002). In combination with neuropsychological measures, olfactory sensory dysfunction thus represents a potential clinical marker of AD severity and progression (Devanand et al., 2015, Wilson et al., 2009, Zou et al., 2016) and could be used for early diagnosis (Wesson et al., 2010).

Apolipoprotein E (ApoE) is a 35 kDa lipid transport protein expressed in various organs, including the brain, where it is especially enriched in the primary olfactory pathway (Mahley, 1988, Nathan et al., 2007). ApoE polymorphic alleles are the main genetic determinants of AD (Liu et al., 2013, Sun et al., 2016). Previous studies have shown ApoE expression in the olfactory nerve, around the glomeruli of the olfactory bulb (OB) (Nathan et al., 2001, Struble et al., 1999) and in the olfactory epithelium (OE) (Nathan et al., 2007). ApoE deficiency (ApoE−/−) leads to significantly delayed recovery, after OE injury, of mature olfactory marker protein positive (OMP+) cells in the OE (Nathan et al., 2010). It also results in delayed olfactory nerve regeneration, along with slow recovery of synaptophysin concentration in the glomerular area of the OB (Nwosu et al., 2008), indicating a vital role of ApoE in nerve repair and remodeling. In addition, ApoE−/− mice performed poorly in olfactory tests (Nathan et al., 2004), suggesting a requirement for ApoE in olfaction. However, the influence of ApoE deletion on olfaction under normal physiological conditions and the mechanisms underlying ApoE regulation of olfaction remain unclear.

In the present study, we evaluated the physiological function of ApoE in the olfactory system by detecting olfactory sensitivity in standard olfactory behavior tests in wild type (WT) and ApoE−/− mice, using a combination of electrophysiological and biochemical techniques. We addressed the following questions: when do olfactory behavioral deficits occur in ApoE−/− mice; and do corresponding early defects occur in the OE, the OB, or both? What molecular and cellular processes lead to olfactory deficits in the absence of ApoE? In addition to uncovering the timing of olfactory behavioral defects in ApoE-deficient mice, the present study reveals new aspects of ApoE involvement in odorant sensation and procession.

Section snippets

Animals and genotyping

ApoE knockout mice (B6.129P2-ApoEtm1Unc/J) on the C57BL/6J background were originally obtained from the Model Animal Research Center of Nanjing University and crossed with C57BL/6J mice to get heterozygous ApoE mice (ApoE+/−). ApoE−/− and WT littermates were generated by intercrossing ApoE+/− males and females as described previously (Zhang et al., 1992). All animals were maintained in a constant temperature (23 ± 1 °C), controlled environment in individual cages with free access to food and

Cookie-finding test reveals impaired olfactory behavior in 3–5 month-old ApoE−/− mice

To investigate when olfactory deficiency caused by the absence of ApoE genes develops, we performed a cookie-finding test (see Fig. 1A), which assesses odorant perception, on 1–2 month- and 3–5 month-old ApoE−/− and WT mice. In this test, the cookie was randomly placed at the end of 1 arm of an 8-arm maze and its position changed for each trial to ensure the animal could not find it using spatial cues. We found that ApoE deficiency resulted in olfactory sensitivity deficiency in 3–5 month-old

Discussion

Despite the involvement of the ε4 ApoE allele in AD pathogenesis (Liu et al., 2013), ApoE induction is beneficial for AD treatment since ApoE inducers restore cognitive abilities in beta-amyloid mouse models (Fitz et al., 2013, Poirier et al., 2014). In this study, in addition to identifying marked differences between 1–2 month- and 3–5 month-old ApoE−/− mice, we used electrophysiological techniques to reveal that the development of olfactory behavioral impairment in ApoE−/− mice corresponds to

Disclosure statement

The authors have no conflicts of interest to disclose.

Acknowledgements

The authors would like to thank Ming Chen for suggestions on the manuscript and Xingzhi Zheng for technical support. This work was supported by grants from the National Natural Science Foundation of China (31771219), the Science and Technology Division of Guangdong Province (2013KJCX0054), the Natural Science Foundation of Guangdong Province (2014A030313418, 2014A030313440), and Guangzhou Science and Technology Bureau (201607010320).

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