Impact of Dietary Fatty Acids on Neuronal and Metabolic Health
In the past decades, the worldwide prevalence of obesity has nearly trebled. This trend is mainly caused by the permanent availability of food, the consumption of a Western-style diet, and the sedentary lifestyle of our modern society. Obesity is a crucial risk factor for the development of several other diseases, such as type 2 diabetes and sporadic Alzheimer’s disease, which altogether represent an enormous burden for modern health care systems. Only recently the importance of the dietary fatty acid (FA) composition for energy balance, functional insulin signalling, and cognitive performances became the focus of research. However, the mechanisms of how different FAs can affect the brain is not yet well understood. Therefore, this thesis firstly aimed to investigate the adverse effects of the saturated FA (SFA) palmitic acid (PA) and the beneficial properties of the omega-3 polyunsaturated FA (ω3-PUFA) docosahexaenoic acid (DHA) on primary hippocampal and cortical cultures isolated from P0/P1 Sprague Dawley rats in vitro. Furthermore, the impact of the in vitro findings was determined in vivo in C57BL/6 mice fed various isocaloric high-fat diets (HFD), enriched either in lard as a natural source of SFAs, New Zealand green-lipped mussel (Perna canaliculus) oil or Hoki (Macruronus novaezelandiae, blue grenadier) liver fish oil, two important sources of ω3-PUFAs, in different concentrations. The second objective was to investigate the role of astrocyte activation and pro-inflammatory cytokine release in the observed results. Finally, this thesis aimed to examine the influence of the dietary fat composition on body weight regulation, energy metabolism and physical activity in young adult and middle-aged mice. Dual-label immunocytochemistry revealed that PA can induce severe, but reversible morphological changes in primary hippocampal and cortical neurons, such as cell body swelling, and axonal and dendritic blebbing. Staining for lipid rafts and β-tubulin suggested that these changes are caused by alterations in the microtubules cytoskeleton and not by shifts in the membrane FA composition. The cytoskeletal rearrangements further caused a reduction in synaptic innervation and compromised neuronal function by reducing the cellular response to low concentrations of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) as demonstrated by calcium-imaging. Intriguingly, the ω3-PUFA DHA was able to prevent all PA-induced morphological changes, if applied simultaneously in equimolar amounts. Since the insulin pathway is known to be altered by HFD feeding in general and PA in particular, the role of insulin signalling in the observed PA-induced effects on neuronal shape was further investigated. Surprisingly, it occurred that augmented insulin signalling rather than insulin resistance caused the morphological changes in neurons as demonstrated by stimulation with different insulin concentrations, and by inhibition of phosphoinositide 3-kinase (PI3K) and glycogen synthase kinase 3β (GSK3β). However, no difference in dendritic spine density or in the gene expression of insulin pathway genes was detected after one week of HFD feeding in the hippocampus of mice, independent of the FA composition of the diet. No signs of astrocyte activation were detected by glial fibrillary acidic protein (GFAP) staining in primary hippocampal and cortical cultures after PA or DHA treatment or in the hippocampus of mice fed one of the various HFDs for one week. Furthermore, no expression of S100 calcium-binding protein B (S100B), tumour necrosis factor α (TNFα) or Apolipoprotein D (ApoD) was detected via in situ hybridisation in the hippocampus, independent of the diet. Accordingly, the level of pro-inflammatory cytokines produced by glial cells was not changed by any treatment or diet as measured by multiplex cytokine immunoassay in the blood of the different diet-fed mice and the supernatants of the primary cultures. Finally, the metabolic studies revealed that high concentrations of New Zealand green-lipped mussel oil can prevent HFD-induced body weight gain in young adult and middle-aged mice if given for 1 to 4 weeks without affecting energy intake. The simultaneous monitoring of metabolic parameters via indirect calorimetry and animal behaviour revealed that total energy expenditure was decreased in young adult, but not in middle-aged mice fed the mussel oil diet. The physical activity and the resting metabolic rate were unchanged, suggesting a difference in the intestinal FA uptake or FA usage by the body.
Advisor: Tups, Alexander
Degree Name: Doctor of Philosophy
Degree Discipline: Physiology
Publisher: University of Otago
Research Type: Thesis