Elsevier

Neuroscience

Volume 343, 20 February 2017, Pages 364-371
Neuroscience

Apolipoprotein E4 causes early olfactory network abnormalities and short-term olfactory memory impairments

https://doi.org/10.1016/j.neuroscience.2016.12.004Get rights and content

Highlights

  • Olfactory deficits are shown in young, but not middle-aged, ApoE4 vs. ApoE3 mice.

  • Young ApoE4 vs. ApoE3 mice exhibit hyperactive LFP response to odors in the OB.

  • Odor-evoked response magnitudes in ApoE4 mice increase in the PCX with age.

  • Spontaneous LFPs in ApoE mice decrease with age in both the OB and the PCX.

  • Spontaneous LFP activity is further reduced in middle-aged ApoE4 vs. ApoE3 mice.

Abstract

While apolipoprotein (Apo) E4 is linked to increased incidence of Alzheimer’s disease (AD), there is growing evidence that it plays a role in functional brain irregularities that are independent of AD pathology. However, ApoE4-driven functional differences within olfactory processing regions have yet to be examined. Utilizing knock-in mice humanized to ApoE4 versus the more common ApoE3, we examined a simple olfactory perceptual memory that relies on the transfer of information from the olfactory bulb (OB) to the piriform cortex (PCX), the primary cortical region involved in higher order olfaction. In addition, we have recorded in vivo resting and odor-evoked local field potentials (LPF) from both brain regions and measured corresponding odor response magnitudes in anesthetized young (6-month-old) and middle-aged (12-month-old) ApoE mice. Young ApoE4 compared to ApoE3 mice exhibited a behavioral olfactory deficit coinciding with hyperactive odor-evoked response magnitudes within the OB that were not observed in older ApoE4 mice. Meanwhile, middle-aged ApoE4 compared to ApoE3 mice exhibited heightened response magnitudes in the PCX without a corresponding olfactory deficit, suggesting a shift with aging in ApoE4-driven effects from OB to PCX. Interestingly, the increased ApoE4-specific response in the PCX at middle-age was primarily due to a dampening of baseline spontaneous activity rather than an increase in evoked response power. Our findings indicate that early ApoE4-driven olfactory memory impairments and OB network abnormalities may be a precursor to later network dysfunction in the PCX, a region that not only is targeted early in AD, but may be selectively vulnerable to ApoE4 genotype.

Introduction

The human APOE gene exists as three alleles – ε2, ε3, and ε4 – which have a worldwide prevalence of 10%, 70%, and 20% respectively (Mahley et al., 2006). Compared to the more common apolipoprotein ε3 allele (ApoE3), the apolipoprotein ε4 allele (ApoE4) is strongly associated with several adverse clinical outcomes, including cardiovascular disease and Alzheimer’s disease (AD) (Farrer, 1997, Mahley et al., 2006, Mahley, 2016). Additionally, ApoE4 has been shown to influence cognitive deficits that arise prior to detectable AD pathology and in healthy older adults (Small et al., 2004, Caselli et al., 2009, Wisdom et al., 2011), suggesting that ApoE4 expression adversely impacts brain pathways independently of AD pathology. In fact, a growing body of neuroimaging research has reported functional differences in human ApoE4 carriers versus noncarriers prior to the onset of cognitive decline within diverse brain regions (such as in hippocampi and specific regions of cortex) (Small et al., 2000, Reiman et al., 2004, Bookheimer and Burggren, 2009, Filippini et al., 2009, Dennis et al., 2010, Sheline et al., 2010, Brown et al., 2011). However, ApoE4-driven functional differences within regions that might specifically explain olfactory dysfunction have largely been ignored.

Prior to broad cognitive decline, human ApoE4 carriers exhibit odor identification impairments (Graves et al., 1999, Murphy, 1999, Wilson et al., 2007, Schubert et al., 2008, Olofsson et al., 2016) that may precede later cognitive deficits (Graves et al., 1999, Wilson et al., 2007, Schubert et al., 2008). Normal olfactory activity relies on proper functioning within the olfactory network, the neural mechanisms of which are increasingly well established (Wilson, 2009): central odor processing initiates within the olfactory bulb (OB), which then transmits information directly to the piriform cortex (PCX), a region that is crucial for procedures involved in odor habituation and olfactory learning (Barnes et al., 2008, Wilson and Linster, 2008, Wilson, 2009, Gottfried, 2010). Olfactory information ultimately enters the entorhinal cortex and finally the hippocampus – a site of memory storage and retrieval (Staubli et al., 1984, Sosulski et al., 2011, Kay, 2014). Olfactory event-related potential (OERP) recordings during olfactory memory tasks in humans have demonstrated differences in peak latencies and amplitudes that distinguish ApoE4 carriers from non-carriers (Corby et al., 2012, Green et al., 2013). However, interpreting the relationship between brain activity and OERP signals measured on the scalp remains challenging (He et al., 2011). Electrode recordings of local field potentials (LFPs) in animal models allow more direct assessment of local activity in circuits directly involved in specific behaviors (Buzsaki et al., 2012). Additionally, odor-evoked LFP oscillations of the rodent olfactory system in the frequencies of 15–40 Hz (beta band; reflects long-range communication) and 40–80 Hz (gamma band; may reflect more local processing) have been shown to be especially indicative of behaviorally relevant odor processing (Neville and Haberly, 2003, Wesson et al., 2011, Kay, 2014, Martin and Ravel, 2014, Sadrian et al., 2014) and are sensitive to neuropathology related to aging (Wesson et al., 2011, Xu et al., 2015). Using LFP recordings from mice that express human ApoE, we have demonstrated within distinct olfactory regions that both ApoE4 genotype and aging play a factor in influencing the signal-to-noise ratio of an odor-evoked response, which may contribute to olfactory memory impairment.

Section snippets

Study approval

All experimental procedures involving animals in this study were approved by and complied with the guidelines of the Institutional Animal Care and Use Committee of the Nathan Kline Institute.

Mice

The mice in this study were purchased from Taconic farms (Germantown, NY, USA) and were homozygous for ApoE4 and ApoE3 on a C57BL/6 background. These targeted-replacement mice were developed to express human ApoE under the control of the endogenous murine promoter (Sullivan et al., 1997), which allows for

Behavioral olfactory deficits in young ApoE4 mice

ApoE mice were initially evaluated using a simple olfactory habituation task. As we have previously described (Wilson, 2009, Wesson et al., 2011), the length of time that a mouse spends investigating novel odors decreases with repeated presentation when olfaction is intact. Analysis of the results by trial (Fig. 1A) with a repeated measures ANOVA (trial × group) on all four groups (6 mo ApoE3, 6 mo ApoE4, 12 mo ApoE3, and 12 mo ApoE4) found a main effect of both trial (F (3,276) = 637.6, p < 0.0001)

Discussion

While previous work has demonstrated impaired olfactory guided behavior in ApoE knock-out mice (Nathan et al., 2004), the present results are the first to investigate specific ApoE genotype effects on olfactory behavior and physiology in vivo. Young (6 mo) ApoE4 compared to age-matched ApoE3 mice display both impaired behavioral odor habituation and OB hyperactivity during odor stimulation, suggesting a relationship between olfactory dysfunction onset and early stage sensory network

Funding

This work was supported by the NIA (P01-AG017617 to PMM and EL; and R01-AG037693 to DAW and EL). Katherine Peng was additionally supported by a training program (T32-GM066704, Dr. Erika Bach) and the Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine. The funding sources were not involved in the study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the article for publication.

Acknowledgments

We thank Dr. Monika Pawlik for her expert assistance with our mouse colonies. KYP did the experimentation and initial data analysis. All authors contributed to the experimental design and to the manuscript.

References (61)

  • J.K. Olofsson et al.

    Odor identification impairment in carriers of ApoE-varepsilon4 is independent of clinical dementia

    Neurobiol Aging

    (2010)
  • J.K. Olofsson et al.

    Long-term episodic memory decline is associated with olfactory deficits only in carriers of ApoE-є4

    Neuropsychologia

    (2016)
  • J. Poirier

    Apolipoprotein E in animal models of CNS injury and in Alzheimer’s disease

    Trends Neurosci

    (1994)
  • B. Sadrian et al.

    Distinct neurobehavioral dysfunction based on the timing of developmental binge-like alcohol exposure

    Neuroscience

    (2014)
  • P.M. Sullivan et al.

    Targeted replacement of the mouse apolipoprotein E gene with the common human APOE3 allele enhances diet-induced hypercholesterolemia and atherosclerosis

    J Biol Chem

    (1997)
  • D.A. Wilson

    Olfaction as a model system for the neurobiology of mammalian short-term habituation

    Neurobiol Learn Mem

    (2009)
  • N.M. Wisdom et al.

    The effects of apolipoprotein E on non-impaired cognitive functioning: a meta-analysis

    Neurobiol Aging

    (2011)
  • W. Xu et al.

    Early hyperactivity in lateral entorhinal cortex is associated with elevated levels of AbetaPP metabolites in the Tg2576 mouse model of Alzheimer's disease

    Exp Neurol

    (2015)
  • A. Arieli et al.

    Dynamics of ongoing activity: explanation of the large variability in evoked cortical responses

    Science

    (1996)
  • D.C. Barnes et al.

    Olfactory perceptual stability and discrimination

    Nat Neurosci

    (2008)
  • P. Berkes et al.

    Spontaneous cortical activity reveals hallmarks of an optimal internal model of the environment

    Science

    (2011)
  • M. Boly et al.

    Baseline brain activity fluctuations predict somatosensory perception in humans

    Proc Natl Acad Sci U S A

    (2007)
  • S. Bookheimer et al.

    APOE-4 genotype and neurophysiological vulnerability to Alzheimer’s and cognitive aging

    Annu Rev Clin Psychol

    (2009)
  • J.H. Brann et al.

    A lifetime of neurogenesis in the olfactory system

    Front Neurosci

    (2014)
  • J.A. Brown et al.

    Brain network local interconnectivity loss in aging APOE-4 allele carriers

    Proc Natl Acad Sci U S A

    (2011)
  • G. Buzsaki et al.

    The origin of extracellular fields and currents–EEG, ECoG, LFP and spikes

    Nat Rev Neurosci

    (2012)
  • R.J. Caselli et al.

    Longitudinal modeling of age-related memory decline and the APOE epsilon4 effect

    N Engl J Med

    (2009)
  • L.A. Farrer

    Effects of age, sex, and ethnicity on the association between apolipoprotein e genotype and alzheimer disease a meta-analysis

    JAMA J Am Med Assoc

    (1997)
  • N. Filippini et al.

    Distinct patterns of brain activity in young carriers of the APOE-epsilon4 allele

    Proc Natl Acad Sci U S A

    (2009)
  • M.D. Fox et al.

    Coherent spontaneous activity accounts for trial-to-trial variability in human evoked brain responses

    Nat Neurosci

    (2006)
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