Docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid, is an essential component of membrane phosphatides and has been implicated in cognitive functions. Low levels of circulating or brain DHA are associated with various neurocognitive disorders including Alzheimer's disease (AD), while laboratory animals, including animal models of AD, can exhibit improved cognitive ability with a diet enriched in DHA. Various cellular mechanisms have been proposed for DHA's behavioral effects, including increases in cellular membrane fluidity, promotion of neurite extension and inhibition of apoptosis. However, there is little direct evidence that DHA affects synaptic structure in living animals. Here we show that oral supplementation with DHA substantially increases the number of dendritic spines in adult gerbil hippocampus, particularly when animals are co-supplemented with a uridine source, uridine-5'-monophosphate (UMP), which increases brain levels of the rate-limiting phosphatide precursor CTP. The increase in dendritic spines (>30%) is accompanied by parallel increases in membrane phosphatides and in pre- and post-synaptic proteins within the hippocampus. Hence, oral DHA may promote neuronal membrane synthesis to increase the number of synapses, particularly when co-administered with UMP. Our findings provide a possible explanation for the effects of DHA on behavior and also suggest a strategy to treat cognitive disorders resulting from synapse loss.

New brain synapses form when a postsynaptic structure, the dendritic spine, interacts with a presynaptic terminal. Brain synapses and dendritic spines, membrane-rich structures, are depleted in Alzheimer's disease, as are some circulating compounds needed for synthesizing phosphatides, the major constituents of synaptic membranes. Animals given three of these compounds, all nutrients-uridine, the omega-3 polyunsaturated fatty acid docosahexaenoic acid, and choline-develop increased levels of brain phosphatides and of proteins that are concentrated within synaptic membranes (e.g., PSD-95, synapsin-1), improved cognition, and enhanced neurotransmitter release. The nutrients work by increasing the substrate-saturation of low-affinity enzymes that synthesize the phosphatides. Moreover, uridine and its nucleotide metabolites activate brain P2Y receptors, which control neuronal differentiation and synaptic protein synthesis. A preparation containing these compounds is being tested for treating Alzheimer's disease.

Although cognitive performance in humans and experimental animals can be improved by administering the omega-3 fatty acid docosahexaenoic acid (DHA), the neurochemical mechanisms underlying this effect remain uncertain. In general, nutrients or drugs that modify brain function or behavior do so by affecting synaptic transmission, usually by changing the quantities of particular neurotransmitters present within synaptic clefts or by acting directly on neurotransmitter receptors or signal-transduction molecules. We find that DHA also affects synaptic transmission in mammalian brain: Brain cells of gerbils or rats receiving this fatty acid manifest increased levels of phosphatides and of specific pre-or post-synaptic proteins. They also exhibit increased numbers of dendritic spines on postsynaptic neurons. These actions are markedly enhanced in animals that have also received the other two circulating precursors for phosphatidylcholine -uridine (which gives rise to brain UTP and CTP), and choline (which gives rise to phosphocholine). The actions of DHA are reproduced by eicosapentaenoic acid (EPA), another omega-3 compound, but not by the omega-6 fatty acid arachidonic acid (AA). Administration of circulating phosphatide precursors can also increase neurotransmitter release (acetylcholine; dopamine) and affect animal behavior. Conceivably, this treatment might have use in patients with the synaptic loss that characterizes Alzheimer's disease or other neurodegenerative diseases, or occurs after stroke or brain injury.

Brain phosphatide synthesis requires three circulating compounds: docosahexaenoic acid (DHA), uridine and choline. Oral administration of these phosphatide precursors to experimental animals increases the levels of phosphatides and synaptic proteins in the brain and per brain cell, as well as the numbers of dendritic spines on hippocampal neurons. Arachidonic acid (AA) fails to reproduce these effects of DHA. If similar increases occur in human brain, giving these compounds to patients with diseases -like Alzheimer's disease -which cause the loss of brain synapses -could be beneficial.

We report that the light-activated bovine metarhodopsin II, upon decay, first forms opsin in the correctly folded form. The latter binds ll1-is-retinal and regenerates the native rhodopsin chromophore. However, when the opsin formed upon metarhodopsin II decay is kept in 0.1% dodecyl maltoside, it converts in a time-dependent manner to a form(s) that does not bind ll-cis-retinal. On subsequent addition of ll-cis-retinal, slow reversal of the non-retinalbinding forms to the correctly folded retinal-binding form has been demonstrated. We have studied the influence, on the above interconversions, of pH, phospholipids (rod outer segment and soybean), dithiothreitol, and a mixture of reduced and oxidized glutathione. Chromophore regeneration in the presence of 11-cis-retinal was highest at pH 6.0-6.3. The addition of dithiothreitol just before bleaching gave back only a small amount (7%) of rhodopsin on the subsequent addition of ll-cis-retinal, whereas the slow phase(s) of chromophore formation was completely abolished. The presence of a mixture of reduced and oxidized glutathione did not significantly affect the results. Addition of phospholipids, either from soybean or rod outer segment, prior to bleaching stabilized the initially formed opsin, resulting in much higher chromophore regeneration. However, addition of the phospholipids after conversion of the opsin to non-retinal-binding form(s) arrested the subsequent reversal of the opsin to the retinalbinding form.Unfolding and refolding of the seven-helix integral membrane protein bacteriorhodopsin was studied in the early 1980s (1). Efficient refolding from a completely denatured state was demonstrated for this protein (2-4). To date, subsequent investigations to refold the dim-light photoreceptor bovine rhodopsin, which contains the specialized three domains, from unfolded states have, however, been uniformly unsuccessful. These attempts constituted a wide variety of conditions including the ideas developed for refolding of bacteriorhodopsin (A. Kronis and H.G.K., unpublished work). We now report on the development of a partially reversible denaturationrenaturation system for this sensory protein. Rhodopsin upon light activation undergoes 11-cis --all-trans isomerization of the retinal chromophore; the metarhodopsin II (Meta II) intermediate formed, after execution of its signal transduction function, discards the bound molecule of all-trans-retinal, and the resulting opsin is believed to recycle by re-forming the functional chromophore by capturing a new molecule of 11-cis-retinal (5). We have now investigated the nature and behavior of the opsin formed from light-activated Meta II in in vitro systems, particularly in the detergent n-dodecyl P3-Dmaltoside (DM).t We find that in the presence of l1-cis-retinal, added immediately after illumination of rhodopsin, the native rhodopsin chromophore regenerates and the rate of its formationThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby ...

Developing neurons synthesize substantial quantities of membrane phospholipids in producing new synapses. We investigated the effects of maternal uridine (as uridine-5′-monophosphate) and docosahexaenoic acid supplementation on pups’ brain phospholipids, synaptic proteins and dendritic spine densities. Dams consumed neither, 1 or both compounds for 10 days before parturition and 20 days while nursing. By day 21, brains of weanlings receiving both exhibited significant increases in membrane phosphatides, various pre- and postsynaptic proteins (synapsin-1, mGluR1, PSD-95), and in hippocampal dendritic spine densities. Administering these phosphatide precursors to lactating mothers or infants could be useful for treating developmental disorders characterized by deficient synapses.

No abstract

No abstract