. Apolipoprotein E, not fibrillar β-amyloid, reduces cerebral glucose metabolism in normal aging. J Neurosci. 2012 Dec 12;32(50):18227-33. PubMed.

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  1. These are intriguing findings.

    It is clear from animal and human studies that ApoE4 has a major effect on Aβ aggregation in the brain, via affecting Aβ clearance and the process of Aβ aggregation itself.

    ApoE may have a variety of other actions in the central nervous system (CNS). The intriguing results here suggest that ApoE4 may be influencing brain glucose metabolism independently of its effect on Aβ aggregation.

    Since the results were all obtained in relatively old individuals (mean age in their seventies), it will be both interesting and important in future studies to determine in large numbers of humans at different ages, especially young adults, whether similar findings are also present. Some studies that are quoted in the discussion of the paper suggest that there are ApoE isoform-related differences in brain activity/metabolism in young adults. It will also be important to verify this in larger sample sets. If there are differences proved early in life, this would provide important insights into how ApoE influences AD and potentially other CNS diseases.

    View all comments by David Holtzman
  2. I daresay, "most intriguing" (referring to the famous Belgian Janssen twins). This could add weight to the Tomm40 implication in AD—but also to "cognitive ageing"?

    References:

    . A genome-wide association study implicates the APOE locus in nonpathological cognitive ageing. Mol Psychiatry. 2012 Dec 4; PubMed.

    View all comments by Fred Van Leuven
  3. There is strong evidence that ApoE interacts with β amyloid to affect its aggregation and clearance, and this may be a major component of ApoE’s role in AD. That said, a number of other potential mechanisms may be involved in ApoE’s contribution to AD, including effects on neurodevelopment and synaptic plasticity. Of the most interest to us has been ApoE's effects on brain energy metabolism, more broadly defined as neuroenergetics.

    In our recent review (1), we explore the links between ApoE and neuroenergetics, drawing on a significant body of brain imaging data and experimental studies using cell culture and animal models. Notably, there are a number of cellular and molecular mechanisms for ApoE to act on energetic processes, including impacts on mitochondrial function and intracellular transport (1,2). Focusing on young adults, brain imaging studies have demonstrated that ApoE4 is associated with FDG-PET measured declines in resting-brain glucose metabolism (3), H2150 PET measured alterations in resting- and task-based cerebral blood flow (4,5), fMRI measured alterations in default-mode network activity at rest and during task activation (6-8), DTI measured reductions in functional anisotropy (9), and potential differences in brain volume measured by MRI (10-12); our earlier study indicated these may be occurring prior to any measurable change in amyloid protein level, plaque deposition, or neurofibrillary tangles (13). We agree with David Holtzman that future study over a wider age range (especially among young adults) will be important in understanding the dynamics of any ApoE effects. Elucidation of the links between ApoE and synaptic activity, brain networks, and neuroenergetics is an intriguing area of ongoing research.

    References:

    . APOE and neuroenergetics: an emerging paradigm in Alzheimer's disease. Neurobiol Aging. 2013 Apr;34(4):1007-17. PubMed.

    . Apolipoprotein e sets the stage: response to injury triggers neuropathology. Neuron. 2012 Dec 6;76(5):871-85. PubMed.

    . Functional brain abnormalities in young adults at genetic risk for late-onset Alzheimer's dementia. Proc Natl Acad Sci U S A. 2004 Jan 6;101(1):284-9. PubMed.

    . APOE related alterations in cerebral activation even at college age. J Neurol Neurosurg Psychiatry. 2005 Oct;76(10):1440-4. PubMed.

    . APOE genotype and cerebral blood flow in healthy young individuals. JAMA. 2003 Sep 24;290(12):1581-2. PubMed.

    . Temporal lobe functional activity and connectivity in young adult APOE varepsilon4 carriers. Alzheimers Dement. 2010 Jul;6(4):303-11. PubMed.

    . Functional magnetic resonance imaging and magnetoencephalography differences associated with APOEepsilon4 in young healthy adults. Neuroreport. 2006 Oct 23;17(15):1585-90. PubMed.

    . Distinct patterns of brain activity in young carriers of the APOE-epsilon4 allele. Proc Natl Acad Sci U S A. 2009 Apr 28;106(17):7209-14. PubMed.

    . The APOE ɛ4 allele modulates brain white matter integrity in healthy adults. Mol Psychiatry. 2011 Sep;16(9):908-16. PubMed.

    . Hippocampal volume differences between healthy young apolipoprotein E ε2 and ε4 carriers. J Alzheimers Dis. 2011;26(2):207-10. PubMed.

    . Influence of brain-derived neurotrophic-factor and apolipoprotein E genetic variants on hippocampal volume and memory performance in healthy young adults. J Neural Transm. 2011 Feb;118(2):249-57. PubMed.

    . Cortical morphology in children and adolescents with different apolipoprotein E gene polymorphisms: an observational study. Lancet Neurol. 2007 Jun;6(6):494-500. PubMed.

    . Reduced posterior cingulate mitochondrial activity in expired young adult carriers of the APOE ε4 allele, the major late-onset Alzheimer's susceptibility gene. J Alzheimers Dis. 2010;22(1):307-13. PubMed.

    View all comments by Jon Valla
  4. The finding that ApoE4 carriers display Aβ-independent pathomechanisms is not really surprising.

    To give a few examples of Aβ-independent effects of ApoE4, the literature shows that ApoE4 carriers also have poor outcome following traumatic brain injury, and have increased risk for HIV-associated dementia, postoperative cognitive dysfunction, and cardiovascular diseases (reviewed in 1). There is significant association between ApoE4 status and poor memory performance in patients with temporal lobe epilepsy (2). Young, healthy ApoE4 carriers display altered functional activation as well as functional connectivity of the medial temporal lobe (3).

    Of course, ApoE4 can exert some effects in Aβ-dependent fashion. This raises a question: What is more important in ApoE4-mediated AD risk, Aβ-independent or Aβ-dependent pathomechanisms?

    A complete understanding of AD pathomechanisms is essential before we can achieve an effective treatment. As I argued previously in the case of TREM2 findings and NSAID data, we must stop interpreting every piece of data through the amyloid lens. The evidence provided by this study further strengthens the merits of that reasoning.

    References:

    . Impact of apoE genotype on oxidative stress, inflammation and disease risk. Mol Nutr Food Res. 2008 Jan;52(1):131-45. PubMed.

    . ApoE-epsilon4 is associated with reduced memory in long-standing intractable temporal lobe epilepsy. Neurology. 2007 Feb 6;68(6):409-14. PubMed.

    . Temporal lobe functional activity and connectivity in young adult APOE varepsilon4 carriers. Alzheimers Dement. 2010 Jul;6(4):303-11. PubMed.

    View all comments by Sanjay Pimplikar

This paper appears in the following:

News

  1. Does ApoE4 Lower Brain Metabolism Independently of Aβ?
  2. Does ApoE4 Risk Begin in the Womb?