Remodeling of lipid bodies by docosahexaenoic acid in activated microglial cells.

Image of a cell containing lipid bodies.

Tremblay M, Zhang I, Disht K, Savage JC, Lecours C, Parent M, Titorenko V, and Maysinger D. (2016) Remodeling of lipid bodies by docosahexaenoic acid in activated microglial cells. Journal of Neuroinflammation, 13(116)

When administered to cell culture or animal models, lipopolysaccharide (LPS; more information can be found here) is used as a proinflammogen to induce an inflammatory response that can include microglial activation, cytokine expression, β-amyloid production, and plasmalogen reduction. Docosahexaenoic acid (DHA; 22:6n3) is an omega-3 polyunsaturated fatty acid that has a variety of biological roles depending on the cell type it is found in and because of this, its function in neural cells is not well understood. However, previous studies indicate that DHA has anti-inflammatory and immunomodulatory characteristics and has been beneficial in improving memory in older adults with mild memory impairments. DHA can be incorporated into the cellular membrane or into lipid bodies (LBs), biological micelles that consist of a lipophilic core containing neutral lipids and a phospholipid surface. Stabilized LBs have important roles in cell function, lipid homeostasis, and are sites for synthesis and storage of inflammatory mediators. Proper function of LBs is crucial, therefore, LB remodelling could be an early indicator of neuroinflammation or neurodegeneration. Tremblay et al analyzed how the incorporation of DHA influences organelle remodeling and its neuroprotective role in N9 microglial cells treated with LPS.

Transmission electron microscopy was used to examine the remodeling of LBs after LPS and/or DHA treatment. There were two types of LBs found in these experiments: lipid vacuoles with a bilayer around the outside and small lipid “droplets” surrounded by a monolayer. When treated with the vehicle, LPS, or DHA the LBs had features of lipid vacuoles, however when the cells were treated with both LPS and DHA fewer were seen. The vacuole size was also treatment dependent: the control cells had small lipid vacuoles, medium were found in the LPS treatment, and large within the DHA. As well, the lipid vacuoles covered the most area in the LPS treatment at ~0.35 μm^2 and the lowest being the control cells at ~0.1 μm^2. DHA was able to reduce the effect of LPS treatment on vacuole area to below 0.2 μm^2. These findings indicate that treatment of DHA causing an increase in LBs size and density associated with LBs reorganization and aggregation.

Any association between LBs and organelles in the microglial cells was also analyzed using TEM. The control or LPS-treated cells had minimal contact between LBs and the mitochondria. Treatment with only DHA caused direct contact between LBs and mitochondria, but when the cells were treated with DHA and LPS many contacts between the two were found. LPS treatment also caused a disturbance in mitochondrial integrity seen through reduced or missing cristae and double membrane, both of which are incredibly important for the function of mitochondria. Interestingly, DHA was able to rescue these effects. Decreased mitochondrial membrane potential is associated with more reactive oxygen species (ROS), therefore Tremblay et al investigated the incorporation of tetramethylrhodamine ethyl ester (TMRE), a red fluorescent dye that is readily taken up by active mitochondria. Although the uptake was reduced by LPS compared to the control treatment, when also treated with DHA the levels were normalized. DHA treatment increased TMRE incorporation and was dependent on dose and time, indicating that mitochondrial function was influenced by the DHA.

LBs have also been shown to interact with the endoplasmic reticulum (ER) in mammalian cell lines and TEM was used to view the effects of DHA on this interaction. Similar to the mitochondria, with control or LPS treatment there were few contacts between the LBs and ER, however after either treatment including DHA there was more contact between the two. As well, in the LPS treatment, the ER lumen was dilated, which can be caused by oxidative stress, but this was not seen often in the control, DHA, or LPS+DHA treatment groups.

Tremblay et al demonstrated the role of DHA in LB remodeling and its ability to recover organelle defects in microglial cells caused by LPS. Although LPS reduced the integrity of mitochondrial membrane and dilated the ER lumen, among other organelle defects, the DHA supplementation was able to return these observations to at least similar levels seen in the control samples. It is unsurprising that increasing the number and size of LBs in a cell would cause different interactions between these components and other organelles, however these findings highlight their benefit. LPS treatments in animals are used to model Alzheimer’s disease (AD) and other neurodegenerative disorders because the associated pathology is similar to that seen in the brains of people with AD. If DHA, or similar polysaturated fatty acids or compounds containing such fatty acids, are able to mitigate some of the negative effects caused by LPS, this may suggest that these lipids could be treatment candidates for improving AD pathology.