Molecular and biophysical features of hippocampal “lipid rafts aging” are modified by dietary n-3 long-chain polyunsaturated fatty acids.

Diaz M, Pereda de Pablo D, Valdes-Baizabal C, Santos G, and Marin R. (2023) Molecular and biophysical features of hippocampal “lipid rafts aging” are modified by dietary n-3 long-chain polyunsaturated fatty acids. Aging Cell

Lipid rafts are regions of lipid membranes that are enriched in lipids such as cholesterol, sphingolipids, saturated acyl chains, and gangliosides. The properties of these types of lipids cause lipid rafts to be a favourable environment for molecular interactions and compartmentalization of protein clusters that regulate signaling pathways. Lipid rafts undergo lipid modifications throughout aging, and these can have an effect on synaptic transmission. This change in lipid rafts associated with aging have been previously shown in the cortex of male mice and was termed “lipid raft aging”, and this has since been demonstrated in the aging human frontal cortex. During lipid raft aging there are changes to lipid structure of the raft bilayer due to a decrease in membrane unsaturation, membrane peroxidability, and cholesterol to phospholipid ratio, as well as increased sphingolipids, saturated fatty acids, and sterol esters. These changes make lipid rafts more viscous and less fluid, which in turn alters their function. The brain also has a high abundance of long-chain polyunsaturated fatty acids (LCPUFAs) and these levels are known to decrease during aging. Diaz et al were interested in investigating changes to lipid levels and physiochemical properties in hippocampal lipid rafts in aged mice and to test whether supplementing with n-3 LCPUFAs recovers or prevents these aging effects.

To accomplish this study, female C57BL/6 mice aged 6 months and 15 months were fed either a typical control diet or one supplemented with n-3 LCPUFAs containing a 2.1:1 eicosapentaenoic acid (EPA) to docosahexaenoic acid (DHA). The 6 month old animals were fed these diets for 6 weeks before analyses and termination and the 15 month old groups was fed these diets for 12 weeks before analyses and termination. First, confirmation that aging alters lipid composition in hippocampal lipid rafts in these animals was completed by extracting lipids and fatty acids from hippocampal lipid raft fractions. The authors found that aging resulted in a significant change to the lipid profiles when comparing the hippocampal raft fractions of 6 month old animals and the 15 month old animals fed the control diets. The 15 month old mice fed the control diet had a significant reduction in DHA, arachidonic acid, LCPUFA, unsaturation index (UI), peroxidability index (PI) compared to the 6 month old groups.

To determine the effect of dietary LCPUFA supplementation, fatty acids in hippocampal lipid rafts were analyzed. Almost no differences were seen between the 6 month old and 15 month old groups that received supplementation. As well, the groups that were supplemented had similar levels to the 6 month old group fed the control diet. The supplementation was shown to restore DHA, levels of saturates, total n-3 LCPUFAs, total PUFAs, as well as UI and PI in lipid rafts when compared to the control fed 15 month old animals. Although, there are many fatty acids that were recovered in the 15 month old supplemented group, but there are some specific species that were still significantly different from the 6 month old groups. These include n-6 LCPUFA arachidonic acid, which had the lowest levels in the supplemented 15 month old group, as well as reduced levels of adrenic acid and docosapentaenoic acid, which are a part of the arachidonic acid biosynthetic pathway.

The lipidome of lipid rafts influences their viscosity and fluidity, and this in turn affects the ability of protein interactions during the initial stages of signaling pathways and for conformational changes that can be required to activate membrane-buried proteins. Interestingly, LCPUFAs are known to be important for this fluidity and because of this the authors wanted to determine how microviscosity and fluidity differed between the treatment groups. This was accomplished using two fluorescent probes, TMA-DPH which targets at the polar head group region and DPH which targets within the hydrocarbon chain core. Microviscosity was higher and the fluidity was lower in the control fed 15 month old group compared to the other groups, while no differences in either were seen between the 6 month old groups and the LCPUFA supplemented 15 month old group. 

Diaz et al were interested in determining the change to lipids and physiochemical properties of lipid rafts in the hippocampus of mice due to aging and whether supplementation with LCPUFAs could help protect from the effects of aging. The authors confirmed a difference in lipid profiles when comparing the control fed 6 month old and 15 month old groups, indicating that there is an effect of aging on lipid rafts. As no difference was seen between the control fed or supplemented 6 month groups, this indicated that the effects of aging had not occurred and these could represent young adults. Interestingly, supplementation with n-3 LCPUFAs for 12 weeks was able to prevent or repair many effects of aging that had occurred or would continue to develop in the supplemented 15 month old animals, with nearly no difference in lipid profile detected between them and the two 6 month old groups. Similarly, microviscosity and fluidity were found to be altered in the control fed 15 month group compared to the other three groups, indicating that aging had a deleterious effect on the two properties, but the LCPUFA supplementation was able to prevent this from occurring. One thing that is important to note with this study is that the supplementation only increases the total polyunsaturated fatty acids in their diet from 2.1% to 2.92%. Although improvements were seen, this is a very modest increase in LCPUFAs, but did provide the only source of EPA and DHA. Further work looking at LCPUFAs and their ability to recover the effects of aging is needed to look at their use for maintaining lipid raft health in old age.