Organism-level experimental data now lend some support to the epidemiologic evidence that the fatty acids in fish oil can protect against Alzheimer disease. Writing in Neuron, a research team led by Greg Cole and Frédéric Calon of the University of California, Los Angeles, reports that removing n-3 polyunsaturated fatty acids (PUFAs) from mouse chow leads to reductions in several significant postsynaptic cytoskeletal proteins, accompanied by learning deficits, in AD transgenic mice. The molecular and behavioral changes could be reversed by adding back the n-3 PUFA docosahexaenoic acid (DHA) to the chow.
Western society is awash in advice to eat certain foods or supplements because they contain fatty acids or antioxidants that may reduce the risk of developing AD. Although a growing body of epidemiological and in-vitro work supports many of these theoretical benefits (see ARF related news story), solid data from live critters, not to mention humans, are needed.
At the heart of the study by Calon and colleagues lies a very astute observation: The carefully crafted chow that laboratory mice eat is chock-full of those n-3 PUFAs that epidemiologic and in-vitro studies have hailed as neuroprotective. Could this help explain, ask the authors, why APP transgenic mice fail to show the deficits in synaptic markers associated with Alzheimer disease?
The authors, swinging the fatty acid content away from n-3 PUFAs, concocted a new, safflower oil-based chow. When they fed this chow to 17-month-old Tg2576 mice, they detected significant changes to levels of postsynaptic structural proteins. For example, dendritic actin levels were reduced, as evidenced by a higher ratio of fractin (a fragment of actin thought to be a caspase cleavage product) to intact actin. Supporting the notion that the postsynaptic actin cytoskeleton is perturbed in the Tg2576 mice, the researchers also found decreases in postsynaptic drebrin. Drebrin is an actin-binding protein concentrated in the brain, especially in dendritic spines, but has been found to be substantially reduced in AD (see, for example, Harigaya et al., 1996). Conversely, and in keeping with previous work, the authors found no corresponding changes in the presynaptic proteins synaptophysin or SNAP-25. Associated with these physiological changes, the learning capacity of the mice in the Morris water maze was significantly reduced.
The good news? Adding DHA to the diet rescued the behavioral deficits, as well as the alterations in postsynaptic actin and drebrin. But how might DHA in the diet have mediated these benefits? Noting that DHA has been shown to activate the neuroprotective phosphotidylinositol-3 (PI3) kinase pathway, which can work via Akt phosphorylation to block caspase activation, the authors assayed this pathway at several points. They found that the n-3 PUFA-depleted chow led to 80-90 percent reduction in one subunit of PI3-kinase, as well as reductions in Akt phosphorylation. DHA supplementation rescued both of these deficits (see ARF related news story on Akt’s involvement in preventing neuronal and muscle damage).
The PI3-kinase pathway links DHA levels to caspase activity, which mediates apoptosis through protein cleavage. Calon and colleagues found that DHA depletion impacts the wonderfully named BAD (for BCL2-antagonist of cell death), which regulates caspases. BAD can be stymied by Akt phosphorylation at BAD's serine 136, and Calon and colleagues found significantly reduced phosphorylation at this BAD residue in the mice fed a DHA-poor diet. DHA supplementation, however, boosted BAD phosphorylation.
The authors even managed to link DHA to the other pillar of nutritional links to AD—oxidative stress. They found evidence that the low n-3 PUFA diet produces more protein carbonyls, markers of oxidative protein damage, another effect that is prevented by DHA in the mouse chow. Which can bring us back to caspases and actin, as the authors propose a model whereby oxidative stress might promote caspase cleavage of actin.
The authors propose a detrimental feed-forward loop in postsynaptic membranes, at least in this mouse AD model, in which oxidative stress is instigated by Aβ and compounded by a diet low in DHA. The oxidative stress further depletes DHA, which is particularly sensitive to lipid peroxidation. An extension of this model posits that without sufficient DHA, activity in the PI3-kinase pathway drops, opening the way for BAD to promote caspase-mediated cleavage of actin. All in all, this is a pretty catastrophic scenario for the postsynaptic membrane.
In an editorial that mostly praises these findings, Lennart Mucke and Robert Pitas of the University of California, San Francisco, do ask for one more bit of work. Noting that lipid manipulations, particularly of cholesterol, impact plaque load and neuronal dystrophy, they suggest that the authors should have assessed these AD hallmarks. "If alterations of DHA in neural membranes affected the production, deposition, or clearance of Aβ, the neuronal alterations observed on the low DHA/PFA diet might represent, at least in part, an Aβ dose effect. Naturally, confirmation of such an effect would not diminish the potential therapeutic significance of the results Calon and colleagues obtained," they write.—Hakon Heimer