Diet is an essential factor in the development of Alzheimer's desease, especially the intake of polyunsaturated fatty acids, such as omega-3. In fact some studies have demonstrated the potential therapeutic use of these molecules in the prevention of this pathology.
Introduction
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that accounts for the major cause of dementia in the world. Considerable attention has been paid to AD disease-modifying factors and risk factors. Cognitive engagement and physical activities have been associated with decreased risk of AD, while diabetes, epsilon 4 allele of the apolipoprotein E gene (APOE4), smoking, and depression have been associated with increased risk of AD. In recent years, increasing evidence supports a role of nutrition in AD. Indeed a dietary pattern, characterized by a high intake of meat, butter, high-fat dairy products, eggs, and refined sugar such as Western Diet contributes to amyloid beta-peptide deposition and oxidative stress. On the other hand the AD risk could be reduced by the consumption of: antioxidants, vitamins (A, E, B, C), polyphenols and omega-3polyinsatured fatty acids. These molecules can be found in vegetables, fish, fruits, coffee, light to moderate alcohol and in Japanese Diet, Health Diets, DASH-Style Diets and Mediterranean Diets.
(Nutrition and the Risk of Alzheimer’s Disease,2013)
The positive effects of nutrients
Antioxidants. The oxidative stress induces the oxidation of biomolecules leading to cellular damage.
Polyphenols. Polyphenols are natural antioxidants that provide protective effects to/for AD through a variety of biological actions, such as interaction with transition metals, inactivation of free radicals, inhibition of inflammatory response, modulation in the activity of different enzymes and effects on intracellular signaling pathways and gene expression. Moreover polyphenols decrease homocysteine plasmatic levels in AD patients.
B Vitamins (Folic Acid, Vitamin B6, and Vitamin B12). B vitamins might might be beneficial to AD by inhibiting oxidative stress and lowering the concentrations of homocysteine.
Polyunsaturated Fatty Acids (Omega-3 Polyunsaturated Fatty Acids). Recent studies suggest that elevated intake of polyunsaturated fatty acids might be beneficial to AD. Dietary supplementation of omega-3 polyunsaturated fatty acids was reported to affect expression of genes that might influence inflammatory process; indeed docosahexaenoic acid (DHA), the main form of omega-3 fatty acids, has been demonstrated to reduce Aβ production and pathological changes in AD animal models.
polyunsatured fatty acid in alzheimer desease
Growing evidence suggests that a high intake of fish is correlated with ta lower risk of developing Alzheimer’s disease (AD) and may delay cognitive decline in patients with mild cognitive impairment (MCI) and very mild AD and it reduces also the symptoms of mood disorders. Higher levels of docosahexaenoic acid (DHA, 22 : 6 n-3) or other omega-3 (n-3) fatty acids (FAs) in the blood have been correlated with a decreased risk of cognitive loss in normal ageing and the development of dementias. Thus, lower levels of DHA and other n-3 polyunsaturated FAs as well as arachidonic acid (AA, 20 : 4 n-6) have been noted in brain phospholipids, lipid rafts and blood lipids in patients affected by AD compared with control subjects.
The OmegaAD study
In 2013 Freund et al. (Transfer of omega-3 fatty acids across the blood–brain barrier after dietary supplementation with a docosahexaenoic acid-rich omega-3 fatty acid preparation in patients with Alzheimer’s disease: the OmegAD study,2013) designed a double-blind, placebo-controlled randomized study: the OmegAD study. In this trial patients with mild or moderate AD have been treated with marine oil containing 2.3 g of n-3 FA (rich in DHA) or placebo daily for 6 months.
CSF FAs
At baseline, there were no significant differences in the cerebrospinal fluid (CSF) concentrations of eicosapentaenoic acid (EPA, 20 : 5 n-3), docosapentaenoic acid (DPA, 22 : 5 n -3), DHA or all n-3 FAs, between the Omega-3 FA and placebo groups. Six months later, EPA, DPA n-3, DHA and all n-3 FAs levels had increased significantly in the Omega-3 group, whereas levels of AA and docosatetraenoic acid and the ratio of n-6/n-3 FAs were lower than in the placebo group.
Plasma FAs
At trial entry, the two treatment groups did not differ significantly with regard to plasma levels of EPA, DPA n-3 and DHA. In the Omega-3 FA group, plasma levels of DHA and EPA as well as all n-3 FAs were significantly higher after 6 months compared with pretrial values, moreover there were significant reductions in the percentages of plasma linoleic acid and AA. These changes were not observed in the placebo group.
CFS vs plasma FAs
Based on the baseline percentage values for all patients, polyunsaturated FA values were generally higher in plasma than in CSF. The difference between plasma and CSF was greater for EPA, followed by linoleic acid, all n-3 FAs and DHA, and least for AA levels.
Marker of AD
There was no significant treatment effect of oral n-3 FA supplementation for 6 months on inflammatory (IL-6 and sIL-1RII) and AD biomarkers (P-tau, T-tau and Aβ 1–42) in CSF and also no effect on inflammatory markers (IL-6, sIL-1RII and high-sensitivity C-reactive protein) in plasma; but changes in CSF levels of biomarkers of AD (P-tau and T-tau) and sIL-1RII significantly correlated with changes of DHA concentrations in CSF. The greater is the increase in DHA, the greater is the increase in sIL-1RII levels but the greater is the decrease in P-tau and T-tau. There were significant correlations between changes over time in CSF concentrations of DHA and in CSF concentrations of P-tau, T-tau and sIL-1RII.
Conclusion
Together, these findings support the hypothesis that increased oral intake of n-3 FAs leads to their accumulation in central nervous tissues and may affect nervous system physiology as well as the pathogenesis and progression of AD, particularly in very mild AD.
DHA & EPA in AD’s Inflammation
Many effects of n-3 FAs are believed to be caused by down-regulation and resolution of inflammation. Alzheimer’s disease (AD) is associated with inflammation mediated by microglia and astrocytes, and n-3 FAs have been proposed as potential remedy for AD. The effect of DHA and EPA on microglial phagocytosis of the AD pathogenic amyloid-β (Aβ) has been analyzed (Omega-3 Fatty Acids Enhance Phagocytosis of Alzheimer’s Disease-Related Amyloid-β42 by Human Microglia and Decreas Inflammatory Markers,2013) on secreted and cellular markers of immune activity, and on the production of brain-derived neurotrophic factor (BDNF). Inflammation is prominent in AD as evidenced by activated microglia and astrocytes, and by increased levels of proinflammatory cytokines in the brain, that results in enhanced production of Aβ.
Dietary supplementation with DHA-rich n-3FAs resulted in increased plasma concentrations of DHA (and EPA) in AD patients. This was associated with reduced release of interleukin IL-1β and IL-6 (pro-inflammatory cytokine).
DHA and EPA are intimately associated with the resolution phase of inflammation. In this final stage, clearance of debris by phagocytosis and secretion of neurotrophic growth factors from activated astrocytes and microglia are activities related to inflammation that can protect and improve the function of neurons.
Effects of n-3 FAs on phagocytosis of Aβ42
DHA and EPA had stimulatory effects on microglial phagocytosis of Aβ42 and it seemed that this stimulation was biphasic, with an immediate peak, followed by a period of relative inertness, and then second peak at 24h. DHA and EPA are precursors for the SPMs (Specialized pro-resolving mediators) which have been shown to stimulate phagocytosis and down-regulate inflammation.
Effects on macrophage polarization
To establish the effect on macrophages polarization phenotype, M1 (CD40 and CD86 markers) and M2 (CD163 and CD206 markers) activation has been investigated. CD163 and CD206 also have a direct link to phagocytosis mediated by their ability to recognise pathogens or debris. Microglia expressing the M1 phenotype markers CD40 and CD86 was significantly decreased by treatment with DHA and EPA. Interestingly, this effect was enhanced upon coincubation with Aβ42. This leads to hypothesize that Aβ42 at the concentration used in Hjorth’s work was not sufficient to evoke a detectable activation, but induced changes which made the microglia more responsive to DHA and EPA.
Effects on infiammatory cytokines
It has been demonstrated that the secretion of the important memory-related neurotrophin brain-derived neurotrophic factor (BDNF)-a protein that normally decreases in AD-patients brain-and of the inflammatory cytokines IL-6 and tumor necrosis factor (TNFα) were increased following DHA and EPA treatment. Incubation of the microglia with DHA resulted in a strong inhibition of the secretion of the proinflammatory cytokine TNFα, whereas the effect of EPA was less prominent. However, these effects of DHA and EPA were only observed upon co-incubation with Aβ 42.
Conclusion
The stimulatory effects of DHA and EPA on microglial phagocytosis of Aβ 42, together with the down-regulatory effects on pro-inflammatory markers, and stimulatory effects on the neuroprotective factor BDNF, indicate that these n-3 FAs activate a pro-resolving program in human microglia.
Therapeutic effects of DHA
Nowadays the only approved drugs for neurological deficits associated with AD are two classes of drugs: acetylcholinesterase inhibitors and NMDA receptor antagonists, even though, their clinical efficacy is very limited. For this reason there is still a high medical need to identify effective treatments.
A possible treatment for Alzheimer’s disease is the administration of docosahexaenoic acid (DHA). DHA is the most abundant n-3 PUFA in the brain and the central nervous system where it involves in neurogenesis, synaptogenesis and synaptic transmission.
An alternative therapeutic approach has been proposed for the treatment of the AD-related neurodegeneration,and it is based on membrane lipid targeting with OHDHA (Membrane lipid modifications and therapeutic effects mediated by hydroxydocosahexaenoic acid on Alzheimer's disease,2013). OHDHA has been administered in double transgenic PS1/APP (5xFAD) mouse model of AD and compared with untreated and WT mice.
OHDHA and endogenous DHA
OHDHA, it is a DHA derivative linked to a hydroxyl group on the α-carbon that blocks its β-oxidation and increases its half-life in lipid membranes. OHDHA is artificially synthesized by Lipopharma Therapeutics (Palma de Mallorca, Spain).
Natural DHA hydroxyl derivates are also produced in the brain, such as neuroprotectin D1 (NPD1), and like DHA, NPD1 is also strongly diminished in the brain of AD patients. The biological function of this molecule has been related to multiple neuroprotective effects, such as antioxidant, anti-inflammatory and anti-apoptotic roles. NPD1 also downregulates Aβ peptide production by modulating β- and α-secretase activities, and it favors neuronal survival against Aβ toxicity.
Moreover DHA is liberated by a stringently regulated phospholipase A2 (PLA2) and is subsequently converted into 10,17S-docosatriene (NPD1) via a 15-lipoxygenase–like (15-LOX–like) enzyme (A role for docosahexaenoic acid–derived neuroprotectin D1 in neural cell survival and Alzheimer disease, 2005).
Effect of OHDHA on membrane lipid composition
OHDHA can modulate the brain membrane lipid composition by increasing the concentration of long chain (40 C) PUFA-containing PE (PhosohatidylEthanolamine) species leading to an increase mainly of diacyl-PE subclasses.
Nowadays the mechanism of OHDHA on membrane lipid composition is still unkown.
Reduction in Aβ levels
The accumulation of Aβ correlates strongly with synaptic degeneration, and with memory and learning deficits. OHDHA-mediated membrane lipid modifications might influence Aβ generation/accumulation; this hypothesis has been tested in 5xFAD mice brains and as expected there was a down-regulation of Aβ levels.
It is supposed that membrane lipids can modulate the amyloidogenic route by acting on BACE-1 complex. OHDHA induces strong clustering of the APP (Amyloid Protein Precursor) and BACE1 proteins into cellular vesicles/organelles suggesting that OHDHA induces membrane lipid changes, that might regulate endolysosomal proteolysis and it could contribute to the downregulation of Aβ. Reduction in Aβ reveals a significant recovery of learning and memory capabilities in OHDHA treated 5xFAD mice. However, the human APP transgene expression and pathway remains unmodified by OHDHA.
Reduction in tau protein phosphorylation
In AD patients, Aβ strongly induces tau phosphorylation. In 5xFAD mice brains tau phosphorylation was statistically increased compared to WT controls. However minimal differences in total tau concentration were found. Therefore the efficacy of OHDHA was tested in a cellular model of neuron-like cells and it was observed a reduction of tau phosphorilation Aβ-mediated.
Neuroprotective effect of OHDHA
OHDHA treatment is also able to protect neuron-like cells against the AD-related toxicity mediated by oligomeric Aβ or NMDA.
These phenomena have been explained examining the response of neuron-like cells incubated with oligomeric Aβ in the presence or absence of OHDHA,; an increase in cellular viability was observed in the first scenario. Similarly the exposure of NMDA-induced Ca2+ excitotoxicity in the presence of OHDHA induced an increase in the number of viable cells. However, the molecular mechanisms connecting OHDHA-mediated membrane lipid changes and NMDA-receptor trafficking at synaptic terminals remain unknown.
OHDHA increases the relative abundance of polyunsaturated lipid species, which might inhibit the interaction of Aβ oligomers with lipid raft-associated protein receptors, thereby inhibiting their cytotoxic cell signaling. This induced an increase in monomeric Aβ that is associated with neuroprotection because it enhances glycogen synthase kinase-3β (GSK-3β) phosphorylation, suggesting that monomeric Aβ may downregulate tau phosphorylation.
Conclusion and future perspectives
AD is a complex neurodegenerative process that has been related to lipid diet and to metabolic diseases. This pathology is probably driven by lipid alterations in neuronal membranes that lead to defective APP processing, altered cell signaling and the resulting characteristic events of this pathology. OHDHA, a “membrane lipid therapy”-based drug, is a molecule designed to target lipid membranes and regulate the membrane-associated protein activity.
In conclusion, OHDHA constitutes a promising therapeutic compound to struggle AD-associated neurodegeneration.
Authors: Melissa Milan, Valeria Rolih, Elisa Rossetti
