Effect of canola oil consumption on memory, synapse and neuropathology in the triple transgenic mouse model of Alzheimer’s disease

The data presented in the current paper demonstrate that chronic administration of a diet enriched with canola oil results in significant deficits of working memory and synaptic pathology, but has no effect on the Aβ deposits and tau phosphorylation levels in a transgenic Alzheimer’s mouse model that develops Aβ deposits and tau neurofibrillary tangles.

Increasing evidence has been accumulated showing that nutritional factors can influence diverse aspects of general health by modulating specific biological systems19. Thus, over the past two decades substantial research has recognized that chronic exposure to the Mediterranean diet is beneficial with respect to reducing the incidence of cardiovascular diseases, and metabolic syndrome20. In addition, longitudinal and prospective clinical trials have revealed that higher adherence to this type of diet is associated with slower rates of cognitive decline, reduced conversion to AD, and improvement of cognitive function21,22. Among the key elements of the Mediterranean diet, an important role has been attributed to daily intake of fresh fruits and vegetables, and the usage of olive oil as a primary source of fat23. In particular, regular daily olive oil consumption has been suggested as the most important and integral component of the diet, and as having a major role in the health benefit of this diet24,25.

This concept has been the propeller for some health organizations in non-Mediterranean countries to promote a Mediterranean diet and the usage of olive oil as the main source of dietary fat. However, this policy has not always been very successful since adopting this type of oil could be more expensive in comparison with other cooking oils in these populations. For this reason, in recent years these countries have been looking for potential alternative to the olive oil. Among them, canola oil has gained increasing attention as a suitable substitute to olive oil especially in countries that lack the primary source for it: the olive tree. As result, canola oil consumption is now quite high in many of these countries because of its lower price compared with olive oil, but also and most importantly because there is a diffuse perception that the canola oil is a healthy choice.

Most of the studies so far investigating the relationship between canola oil consumption and health benefits have shown limited evidence of beneficial effects or neutral action on biomarkers of risk factors for cardiovascular diseases26. On the other hand, studies have provided conflicting results depending on the experimental model implemented, the length of the treatment and the particular end-point considered27,28. However, no data are available on the biological effects that chronic exposure to dietary canola oil may have on cognitive function and the development of the AD-like phenotype which typically include: memory, synaptic integrity, Aβ and tau neuropathology.

To address this scientific question, we implemented a dietary approach and utilized a transgenic mouse model, the 3xTg mice, which manifest all these aspects including memory impairments, Aβ deposits and tau tangles pathology16.

First, we observed that compared with 3xTg mice receiving regular chow diet, the group treated with canola oil-rich diet had a significant increase in body weight suggesting that the added oil provided extra calories to the mice. This observation is in contrast with previous reports showing that chronic diet supplementation with canola oil had no effect on the average animal body weight28,29. We interpret this discrepancy as secondary to the different strains of mice that were implemented in those studies, and probably the length of our study.

However, this fact did not translate in any alteration of their motor ability since we did not observe any differences between the two groups when for instance the animals were tested in the different behavioral paradigms. Thus, in the Y-maze no significant differences were observed between the two groups when the number of entries in each arm of the maze was considered suggesting that the diet and the higher body weight did not alter the motor ability of the mice. By contrast, compared with 3xTg kept on a regular chow diet, the ones receiving canola oil-supplemented diet had a significant reduction in the percentage of spontaneous alternations in the Y-maze, suggesting an impairment of their working memory30.

Supporting the detrimental effect of chronic exposure to canola oil-rich diet on the behavior responses, we found that the same mice had biochemical evidence for a reduction in synaptic integrity as demonstrated by the significantly lower levels of PSD95 protein, a well-established synaptic marker, in the brains of the canola oil-treated mice31.

Analysis of the amount of Aβ 1-40 and Aβ 1-42 peptides in the soluble fractions from brain cortices of these mice did not show any significant differences between the two groups. A similar result was obtained when we assayed the formic acid soluble fraction of the Aβ 1-42 peptides. By contrast, we observed that brain samples from mice treated with the canola oil had a significant reduction the formic acid soluble fraction of Aβ 1-40 peptides, which is considered less prone to precipitate and form insoluble deposits compared to the Aβ 1-42 peptides32. Normally Aβ 1-40 is produced at higher levels, but as Aβ 1-42 is more hydrophobic and has a stronger tendency to polymerize into neurotoxic species, it seems to be of particular importance in AD pathogenesis33,34. This is supported by studies on mutations in APP, presenilin 1 (PSEN1) and PSEN2, which show an increased Aβ 42/40 ratio35. Interestingly, an analysis of the ratios among the two fractions of Aβ peptides revealed that the brains of the mice receiving canola oil had a statistically significant increase in the ratios of Aβ 42/40 suggesting a shift towards the more prone to fibril formation and insoluble form of these peptides, which would favor their progressive precipitation and intracellular accumulation.

This observation has great biologic importance, since data in the literature have shown that from a mechanistic point of view elevation in Aβ 42/40 peptide ratio enhances the nucleation and fibrillogenesis of pathogenic Aβ 1-42 peptides, events that are otherwise compromised by the presence of high levels of secreted Aβ 1-40 peptides36. Confirming this aspect of the Aβ peptides neurobiology, we found a trend towards an increase in the amount of Aβ deposits immunoreactivity in the brains of the canola oil-treated mice compared with controls.

No significant effect of the canola oil-rich diet was found on some of the major protein systems in place to control Aβ clearance and degradation. Thus, steady state levels of apoE, a major Aβ chaperone, levels of neprilysin and IDE, two major Aβ catabolic pathways, were no different between the controls and canola oil-treated mice.

Since this model is known to develop high levels of phosphorylated tau protein and ultimately forms neurofibrillary tangles, next we were very interested in assessing whether our dietary treatment had any influence on this aspect of their phenotype. By the end of the chronic treatment, levels of total soluble tau and different phosphorylated isoforms were undistinguishable between the two groups, suggesting that canola oil does not influence tau metabolism.

Since previous works have shown that olive oil has a potent anti-inflammatory action in vivo, next we assessed the effect of chronic canola oil exposure on classical biomarkers of activation for microglia and astrocytes, two major cellular components and modulators of neuroinflammatory responses37. Brain homogenates from canola oil treated mice were not different from the ones receiving chow diet controls when the steady state levels of GFAP, a marker of astrocytosis, and IBA1, a marker of microglia activation, were measured.

Finally, since we previously reported that olive oil is an activator in vivo of the autophagic machinery15, we also investigated whether or not this was the case in the mice receiving the canola oil-rich diet. Assessment of several well-established markers of autophagy activation in the brain of the two groups of mice did not show any significant differences, suggesting that canola oil does not influence this system38.

In conclusion, our investigation demonstrates for the first time to the best of our knowledge a negative effect of the chronic consumption of canola oil on memory, synaptic integrity and Aβ 42/40 ratios in a mouse model of AD. The translational value of our findings lies in the observation that this type of oil supplementation can influence some of the most important features of the AD pathological phenotype.

Overall our findings do not provide support to some of the current ideas suggesting healthy benefits deriving from the regular consumption of canola oil. Although we recognize that more studies are needed to investigate the biological effects of this oil, our data would not justify the increasing tendency of replacing olive oil with canola oil as part of a good and healthy dietary alternative in non-Mediterranean countries.