Elsevier

Behavioural Brain Research

Volume 348, 1 August 2018, Pages 90-100
Behavioural Brain Research

Biochemical and cognitive effects of docosahexaenoic acid differ in a developmental and SorLA dependent manner

https://doi.org/10.1016/j.bbr.2018.04.017Get rights and content

Highlights

  • Dietary omega-3 supplementation/deprivation effect requires lifelong exposure.

  • Lifelong DHA exposure reduces spatial learning and memory in SorLA deficient mice.

  • Reduced antioxidant capacity of vitamin C in brains of SorLA deficient mice.

Abstract

Beneficial effects of omega-3 fatty acid intake on cognition are under debate as some studies show beneficial effects while others show no effects of omega-3 supplementation. These inconsistencies may be a result of inter-individual response variations, potentially caused by gene and diet interactions. SorLA is a multifunctional receptor involved in ligand trafficking including lipoprotein lipase and amyloid precursor protein. Decreased SorLA levels have been correlated to Alzheimer’s disease, and omega-3 fatty acid supplementation is known to increase SorLA expression in neuronal cell lines and mouse models. We therefore addressed potential correlations between Sorl1 and dietary omega-3 in SorLA deficient mice (Sorl1−/−) and controls exposed to diets supplemented with or deprived of omega-3 during their entire development and lifespan (lifelong) or solely from the time of weaning (post weaning).

Observed diet-induced effects were only evident when exposed to lifelong omega-3 supplementation or deprivation as opposed to post weaning exposure only. Lifelong exposure to omega-3 supplementation resulted in impaired spatial learning in Sorl1−/− mice. The vitamin C antioxidant capacity in the brains of Sorl1−/− mice was reduced, but reduced glutathione and vitamin E levels were increased, leaving the overall antioxidant capacity of the brain inconclusive. No gross morphological differences of hippocampal neurons were found to account for the altered behavior.

We found a significant adverse effect in cognitive performance by combining SorLA deficiency with lifelong exposure to omega-3. Our results stress the need for investigations of the underlying molecular mechanisms to clarify the precise circumstances under which omega-3 supplementation may be beneficial.

Introduction

Omega-3 and omega-6 fatty acids are both polyunsaturated fatty acids (PUFAs) that are essential structural components of cell membranes and can also act as mediators of inflammation. Long-chain omega-3 and omega-6 fatty acids can be synthesized from α-linolenic acid (ALA, 18:3, n-3) and linoleic acid (LA, 18:2, n-6), respectively, but conversion rates are very slow, and consequently, dietary intake of long-chain PUFAs is necessary (Fig. 1). Omega-3 fatty acids have been shown to be essential for a healthy cognitive status and are of particular importance during neurodevelopment, and omega-3 fatty acid deficiency during pregnancy and infancy has been linked to neurodevelopmental disorders such as autism and attention-deficit hyperactive disorder (ADHD) [[1], [2], [3]]. Moreover, omega-3 fatty acid deficiency has been associated with decreased cognitive performance in relation to ageing and neurodegenerative diseases like Alzheimer’s disease [[4], [5], [6]].

The omega-3 fatty acid docosahexaenoic acid (DHA 22:6, n-3) is of particular interest as it comprises 10–20% of total brain lipids and 90% of total omega-3 PUFAs in the brain [7,8]. Neurodevelopmental disorders such as autism and ADHD are often associated with decreased neuronal plasticity [9,10], which DHA may alleviate by promoting neurite outgrowth, synaptogenesis and differentiation of neural stem cells [[11], [12], [13], [14], [15]]. A recent study on rats suggested that impaired hippocampal-dependent memory may be related to decreased neurogenesis [30] and DHA deficiency has likewise been shown to decrease performance of mice in spatial memory tasks possibly influenced by decreased plasticity [31]. Together, these findings suggest that DHA may be a key player in various cognitive health aspects.

In addition to structural functions, omega-3 and omega-6 fatty acids are involved in inflammation. As omega-3 and omega-6 fatty acids are competitively converted into anti-inflammatory or pro-inflammatory eicosanoids, respectively, a low intake of omega-3 fatty acids shifts the balance towards products facilitating neuroinflammation (Fig. 1) [32]. Children with autism have been shown to display increased levels of neuroinflammation and oxidative stress, underlining the importance of the omega-3 and omega-6 fatty acid intake [[33], [34], [35], [36]].

Supplementation studies with omega-3 fatty acids in cellular experiments as well as in animal and human studies suggest a beneficial effect of omega-3 fatty acid supplementation in cognitive health as well as in protection against neurodegenerative diseases, as summarized in [16,17]. However, the reported beneficial effects are being challenged by several randomized controlled clinical trials and Cochrane reviews showing little or no effect [[18], [19], [20], [21]]. A contributor to these conflicting results may be genetic differences among individuals, and increased attention is being directed towards effects of diet–gene interactions. Gender and specific enzyme polymorphisms involved in synthesis of long-chain PUFAs have both been shown to affect the omega-3 fatty acid status of test subjects [22,23]. Therefore, to determine the value of omega-3 fatty acid supplementation, systematic studies on dietary fatty acid interaction with the genetic makeup, i.e. the individual’s genetic composition, are needed.

SorLA is a multifunctional 250 kDa type-I receptor that belongs to the family of Sortilin receptors, also known as Vps10p-D receptors. SorLA, which is encoded by SORL1 and is highly expressed in the brain, mediates sorting of ligands between the trans-Golgi network, the cell surface and the endosomes and has recently been shown to mediate transcytosis of lipoprotein lipase (LpL) [[24], [25], [26], [27], [28]].

The SorLA ligand LpL hydrolyzes dietary PUFAs as they enter the brain and recent studies in mice have indicated that LpL, through yet unidentified mechanisms, regulates PUFA availability in the CNS consequently affecting synaptic plasticity [33]. Furthermore, lack of LpL in the mouse brain has been found to correlate with impaired learning and memory and with a low brain vitamin E status, which could be reversed by omega-3 fatty acid supplementation [34]. Reduced SorLA levels have been observed in patients with AD, and the omega-3 fatty acid DHA has been found to increase SorLA levels in various cell lines as well as in mouse models [[29], [30], [31]] suggesting that DHA may have a beneficial effect on patients with AD. SorLA also has a more direct effects on lipid metabolism in that soluble SorLA has been shown to repress the thermogenesis in adipose tissue [32].

Lack of either LpL or SorLA has been shown to result in impaired cognition, and DHA supplementation has been found to rescue the effects of LpL deficiency as well as to increase SorLA levels. Together, these findings are strong indications of an interplay between SorLA, LpL and omega-3 fatty acids in regard to cognitive functions. Based on the ability of SorLA to mediate LpL transcytosis, we hypothesized that SorLA deficiency may lead to altered LpL regulation of PUFA availability. Altered PUFA availability may thus lead to neuronal morphology changes and to altered inflammatory and anti-oxidative status in the brain ultimately affecting behavioral outcomes. Thus, variations in individual genetic makeup of the gene encoding SorLA may contribute to the conflicting results of omega-3 fatty acid supplementation studies.

In this study, we investigate the interplay between SorLA and PUFAs by focusing on the effects of omega-3 fatty acid supplementation and deprivation at various time points in life in combination with presence or absence of SorLA by assessing biochemical, neuroanatomical and behavioral consequences.

Section snippets

Animals, diets and study designs

All mice were bred and maintained at the in-house breeding facility (Department of Biomedicine, Aarhus University, Denmark). SorLA knockout mice (Sorl1−/−) on C57VL/6 J background were backcrossed for more than 10 generations to obtain a congenic line [35].

Mice were kept in standard enriched environment cages with 2–5 mice/cage, with EnviroDri nesting material, steel shelter, 2 small wood blocks and a paper tissue and were provided with food and water ad libitum. The environment was temperature

Anxiety

General anxiety levels were assessed in the elevated plus maze, where reduced exploration of the open arms of the maze is considered to reflect increased anxiety. All mice, independent of genotype, diet and timing of exposure showed a preference for the closed arms, indicated by less than 50% time spent or 50% of entries onto the open arms, reflecting overall normal behavior. Lifelong Ω3 diet exposure decreased the time spent on the open arms for both genotypes compared to mice fed Ω6 diet (F(

Discussion

Studies of PUFA-gene interactions have so far mainly focused on genes related to PUFA synthesis, e.g. FADS1 and FADS2 [23,40,41]. In this study, we investigated the interplay between PUFAs and the gene encoding SorLA, a gene previously reported to have an effect on fatty acid levels and on fatty acid metabolism. Wild type and SorLA deficient mice were exposed to an omega-3 fatty acid deficient (Ω6) or DHA-enriched diet (Ω3) either from the time of conception and throughout life (lifelong) or

Conclusions

The present study not only links dietary effects with genetic makeup but also with the timing of dietary supplementation onset as (1) SorLA deficient mice displayed decreased spatial learning with lifelong exposure to DHA-enriched diet and (2) post weaning exposure resulted in reduced spatial learning in SorLA deficient mice regardless of diet. We observed an increase of total α+γ-tocopherol levels in the brain of mice with lifelong exposure to DHA-enriched diet independent of genotype. As we

Acknowledgements

This work was supported by Research Initiative on Brain Barrier and Drug Delivery funded by the Lundbeck foundation to MSN (Grant no. 2013-14113) and SG (R198-2015-168). The authors declare no conflict of interests.

We would like to thank Annie B. Kristensen, Joan Frandsen and Belinda Bringtoft for excellent technical assistance.

References (85)

  • L. Jacobsen et al.

    Activation and functional characterization of the mosaic receptor SorLA/LR11

    J. Biol. Chem.

    (2001)
  • Y. Motoi et al.

    Neuronal localization of a novel mosaic apolipoprotein E receptor, LR11, in rat and human brain

    Brain Res.

    (1999)
  • J. Lykkesfeldt

    Determination of ascorbic acid and dehydroascorbic acid in biological samples by high-performance liquid chromatography using subtraction methods: reliable reduction with tris[2-carboxyethyl]phosphine hydrochloride

    Anal. Biochem.

    (2000)
  • P.J. Hissin et al.

    A fluorometric method for determination of oxidized and reduced glutathione in tissues

    Anal. Biochem.

    (1976)
  • L.G. Gillingham et al.

    Dietary oils and FADS1-FADS2 genetic variants modulate [13C]alpha-linolenic acid metabolism and plasma fatty acid composition

    Am. J. Clin. Nutr.

    (2013)
  • I. Fedorova et al.

    An n-3 fatty acid deficient diet affects mouse spatial learning in the Barnes circular maze

    Prostaglandins Leukot. Essent. Fatty Acids

    (2007)
  • I. Carrie et al.

    Phospholipid supplementation reverses behavioral and biochemical alterations induced by n-3 polyunsaturated fatty acid deficiency in mice

    J. Lipid Res.

    (2000)
  • W.L. Chung et al.

    Fish oil supplementation of control and (n-3) fatty acid-deficient male rats enhances reference and working memory performance and increases brain regional docosahexaenoic acid levels

    J. Nutr.

    (2008)
  • P.E. Wainwright et al.

    Water maze performance is unaffected in artificially reared rats fed diets supplemented with arachidonic acid and docosahexaenoic acid

    J. Nutr.

    (1999)
  • J.P. Pan et al.

    Some subtypes of endocannabinoid/endovanilloid receptors mediate docosahexaenoic acid-induced enhanced spatial memory in rats

    Brain Res.

    (2011)
  • B.M. Ross et al.

    Dietary omega-3 polyunsaturated fatty acid supplementation in an animal model of anxiety

    Prostaglandins Leukot. Essent. Fatty Acids

    (2016)
  • H. Frances et al.

    Effects of dietary alpha-linolenic acid deficiency on neuromuscular and cognitive functions in mice

    Life Sci.

    (1995)
  • J.P. SanGiovanni et al.

    The role of omega-3 long-chain polyunsaturated fatty acids in health and disease of the retina

    Prog. Retinal Eye Res.

    (2005)
  • K. van Elst et al.

    Food for thought: dietary changes in essential fatty acid ratios and the increase in autism spectrum disorders

    Neurosci. Biobehav. Rev.

    (2014)
  • F. Gu et al.

    Impaired synthesis and antioxidant defense of glutathione in the cerebellum of autistic subjects: alterations in the activities and protein expression of glutathione-related enzymes

    Free Radic. Biol. Med.

    (2013)
  • J. Lykkesfeldt et al.

    Vitamin C

    Adv. Nutr.

    (2014)
  • O. Yorbik et al.

    Investigation of antioxidant enzymes in children with autistic disorder

    Prostaglandins Leukot. Essent. Fatty Acids

    (2002)
  • S. Smesny et al.

    Effects of omega-3 PUFA on the vitamin E and glutathione antioxidant defense system in individuals at ultra-high risk of psychosis

    Prostaglandins Leukot. Essent. Fatty Acids

    (2015)
  • D. Goti et al.

    Effects of lipoprotein lipase on uptake and transcytosis of low density lipoprotein (LDL) and LDL-associated alpha-tocopherol in a porcine in vitro blood-brain barrier model

    J. Biol. Chem.

    (2002)
  • E. Gumpricht et al.

    Can omega-3 fatty acids and tocotrienol-rich vitamin E reduce symptoms of neurodevelopmental disorders?

    Nutrition (Burbank, Los Angeles County, Calif.)

    (2014)
  • S. Glerup et al.

    SorLA controls neurotrophic activity by sorting of GDNF and its receptors GFRalpha1 and RET

    Cell Rep.

    (2013)
  • H. Kasai et al.

    Structural dynamics of dendritic spines in memory and cognition

    Trends Neurosci.

    (2010)
  • O. Ben-Zeev et al.

    Synthesis and regulation of lipoprotein lipase in the hippocampus

    J. Lipid Res.

    (1990)
  • L. Dagai et al.

    Docosahexaenoic acid significantly stimulates immediate early response genes and neurite outgrowth

    Neurochem. Res.

    (2009)
  • B. Koletzko et al.

    The roles of long-chain polyunsaturated fatty acids in pregnancy, lactation and infancy: review of current knowledge and consensus recommendations

    J. Perinat. Med.

    (2008)
  • R. Uauy et al.

    Nutrition in brain development and aging: role of essential fatty acids

    Nutr. Rev.

    (2006)
  • A.L. Petursdottir et al.

    Effect of dietary n-3 polyunsaturated fatty acids on brain lipid fatty acid composition, learning ability, and memory of senescence-accelerated mouse

    J. Gerontol. Ser. A Biol. Sci. Med. Sci.

    (2008)
  • M.C. Morris

    Docosahexaenoic acid and Alzheimer disease

    Arch. Neurol.

    (2006)
  • C. Piochon et al.

    LTD-like molecular pathways in developmental synaptic pruning

    Nat. Neurosci.

    (2016)
  • F. Calderon et al.

    Docosahexaenoic acid promotes neurite growth in hippocampal neurons

    J. Neurochem.

    (2004)
  • D. Cao et al.

    Docosahexaenoic acid promotes hippocampal neuronal development and synaptic function

    J. Neurochem.

    (2009)
  • S.C. Dyall

    Long-chain omega-3 fatty acids and the brain: a review of the independent and shared effects of EPA, DPA and DHA

    Front. Aging Neurosci.

    (2015)
  • View full text