The recognition that nutrient intake can affect the rate at which the brain makes new synapses arose largely from basic-science studies performed in MIT’s Department of Brain and Cognitive Sciences between 1999 and 2012 [1
]. As described below, these studies showed that when three nutrients—uridine (as its monophosphate, UMP); the omega-3 fatty acid DHA (or EPA, which is equally effective [4
]); and choline—are consumed together, brain cells produce more of the specialized membranes [1
] used for forming dendritic spines [2
], the immediate anatomic precursor of synapses. The nutrients also improve cognitive test scores in both impaired and normal experimental animals [2
]. Based on these findings, and on the consensus that a deficiency in synapses underlies the memory disorder seen in early Alzheimer’s disease [5
], a mixture containing the three nutrients plus others that enhance their efficacy, SOUVENAID®
was formulated and tested, and found to improve memory scores [6
] and the connectivities between brain regions [7
] among patients with early Alzheimer’s Disease.
The ability of the brain to make more phosphatidylcholine (PC), and related membrane constituents when presented with additional uridine, DHA, and choline, requires that the enzymes which catalyze the steps in PC synthesis have a special biochemical property: Each enzyme must exhibit unusually poor affinity for its substrate, so that, at the uridine, DHA, and choline concentrations normally present in brain cells, many enzyme molecules will not be attached to their substrate and, thus, will not be able to act on it. However, when additional substrate is provided—by administering the nutrients to raise their blood levels—more enzyme molecules become engaged, and more of their product is formed [2
]. We had first observed a similar “precursor-dependence” many years earlier involving the conversion of a different nutrient, the amino acid tryptophan, to serotonin, a neurotransmitter [8
]: Giving animals tryptophan rapidly increased the formation and release of brain serotonin. Similar precursor-dependence was subsequently shown to characterize numerous other neurotransmitters [9
The concentrations of uridine, DHA, and choline required for engaging just half of the molecules of each enzyme needed for converting choline to PC (i.e.
, each enzyme’s Michaelis-Menten constant [Km]) were found to be very high in relation to their actual brain concentrations. Hence, administering each of the nutrients does in fact increase the production, and, ultimately, the levels of brain PC [1
]. Moreover, the effect of giving uridine, DHA, and choline together is additive or greater. An additional biochemical mechanism activated by the uridine in SOUVENAID®
enables the brain also to increase production of the special proteins found in synaptic membranes. It involves the activation of P2Y receptors, a particular family of brain receptors that can affect neuronal differentiation and synaptic protein synthesis [10
]. P2Y receptor activation is also involved in the processes that shape the newly-formed membrane into neurites, dendritic spines, and synapse [10
2. Control of Synaptogenesis
Most of the communications that take place between a brain neuron that transmits a signal and one that receives that signal occur at synapses. These are highly specialized structures, which include a presynaptic terminal budding from the transmitting neuron’s axon; the postsynaptic membrane, usually on one of the receiving neuron’s dendrites; and the space between the neurons—the synaptic cleft. Presynaptic terminals make and store the neuron’s neurotransmitter, and release some of it into the synaptic cleft when the neuron is depolarized. The postsynaptic membranes contain receptors to which the neurotransmitter can then attach, as well as other protein molecules which mediate the functional consequences of receptor activation. Pre- and post-synaptic membranes are composed principally of phosphatides like PC; other phospholipids and cholesterol; and particular proteins, many of which are confined to synaptic structures.
The postsynaptic membranes on which glutamate, the most widely-used excitatory brain neurotransmitter, acts contain large numbers of dendritic spines, as well as postsynaptic densities (PSD) which house various proteins that mediate most of the functional consequences of receptor activation. For example, some of these proteins open or close “pores” in the membranes, thereby allowing specific ions which affect the cell’s voltage to pass in or out; others activate enzymes that synthesize second-messenger compounds (like cyclic-AMP or diacylglycerol), which affect gene expression and the overall metabolism of the post-synaptic neuron.
The formation of a new synapse (by, for example, hippocampal neurons that use glutamate as their neurotransmitter) can be initiated by the coming together of what will become a presynaptic terminal and a dendritic spine. Thus the availability of dendritic spines is an important factor controlling the rate of synaptogenesis. This number can be increased by various treatments (e.g., by giving the hormone ghrelin—which also enhances memory performance and long-term potentiation, a form of learning), or decreased by Alzheimer’s and other dementia-producing diseases. The sparsity of dendritic spines in AD brain [11
] probably causes a decrease in the formation of new synapses. This may partially explain the decreased number of cortical synapses and the consequent memory impairment observed early in the course of the disease.
Although most brain synapses are formed prenatally or during early postnatal development, each survives for only days to months, and must be renewed periodically throughout the individual’s life span. This continuing necessity is probably a major factor underlying the brain’s plasticity and the individual’s ability to learn, since it allows specific, newly-formed synapses to be associated with newly-learned material. Early in development, most synaptogenesis occurs in the absence of neuronal depolarization and neurotransmitter release. In adulthood, however, the rate at which new synapses form, and the ways their connections become configured, are largely governed by neuronal activity. This allows very active synapses to facilitate the formation of additional functionally-related synapses. However, the magnitude
of the increase in synaptogenesis among active neurons is apparently modulated by nutrient availability, specifically of uridine, DHA, and choline [1
3. Biosynthesis of Membrane Phosphatides, Synaptic Proteins, Neurites, and Dendritic Spines: Effects of Uridine, Dha, and Choline
3.1. Membrane Phosphatides
All cells utilize DHA and other fatty acids (e.g., EPA); uridine; and choline to form the phosphatide compounds that constitute the major components of their membranes. PC, the most abundant phosphatide in brain, is synthesized from these precursor-nutrients by a set of enzymes that comprise the CDP-choline cycle (or “Kennedy Cycle
”). This biochemical pathway also generates a related membrane constituent, the phosphatide phosphatidylethanolamine (PE). Phosphatidylserine (PS), the third major membrane phosphatide, is produced by exchanging a serine molecule for the choline in PC or the ethanolamine in PE. Sphingomyelin, the other major choline-containing brain constituent, is formed from PC. Thus, all of the principal lipid components of synaptic membranes are affected by the rate at which PC is being formed. In addition, since each of the reactions needed to convert choline, uridine, and DHA to PC is catalyzed by a low-affinity enzyme, blood levels of the three nutrient-precursors can determine not only PC’s rate of synthesis but also the rates at which almost all of the brain’s membrane lipids are produced. When all three of the nutrients are provided concurrently the resulting increase in PC production is greater than the sum of the increases produced by giving each separately [1
]. This probably occurs because if just one of the nutrient-precursors were to be provided, the concentrations of the other two would continue to be limiting.
The CDP-choline cycle involves three sequential enzymatic reactions [2
]: In the first, catalyzed by choline kinase (CK), a phosphate group is transferred from ATP to the hydroxyl oxygen of choline, yielding phosphocholine. The second reaction, catalyzed by CTP:phosphocholine cytidylyltransferase (CT), transfers cytidine-5′-monophosphate (CMP), a portion of the high-energy CTP molecule, to the phosphocholine, yielding cytidine-5′-diphosphocholine (also known as CDP-choline, or as citicoline). (As discussed below, most of the CTP that the human brain uses for PC synthesis comes from circulating uridine, there being little or no free cytidine in human blood) [12
]. The third and last reaction, catalyzed by the enzyme CDP-choline:1,2,diarylglycerol cholinephosphotransferase (CPT), transfers the choline moiety from CDP-choline to the free hydroxyl group of diacylglycerol (DAG), yielding the PC. As described below, the brain obtains all three of these PC precursors mostly or entirely from the blood stream.
Thus, administering choline increases brain phosphocholine levels in rats and humans because CK’s Km for choline (2.6 mM) is much higher than usual brain choline levels (30–60 µM) [2
]. Similarly, administering uridine increases brain levels of UTP and then CTP [2
], and giving DHA increases the concentration of DAG molecules likely to be used for production of PC (because one of their two fatty acids happens to be DHA, an omega-3 compound).
3.2. Proteins Localized in Pre- and Post-Synaptic Membranes
Administration together of DHA, uridine (as UMP) and choline also increases the levels of proteins that are localized within presynaptic and postsynaptic membranes (for example, synapsin-1P; syntaxin-3; PSD-95), but not of other proteins, like beta-tubulin, which are found all over the brain [1
]. The increase in synaptic proteins probably results from the additional action of uridine described above, i.e.
, its ability (shared with uridine-containing nucleotides like UTP) to activate P2Y receptors in the brain [10
]. Nucleotides like UTP activate a variety of receptor subtypes that stimulate either ion fluxes (P2X) or intracellular metabolic processes (P2Y). The P2Y receptors in brain (P2Y2; P2Y4; P2Y6) tend to be specific for uridine-containing compounds. Their activation increases intracellular concentrations of the second-messengers DAG, inositol triphosphate, and calcium.
3.3. Neurite Outgrowth
Activation of P2Y receptors by uridine or UTP also affects the outgrowth, from neuron-derived cells, of neurites [10
] (which, like dendritic spines, can be precursors for synapses, and are principally composed of membranes). P2Y activation can also stimulate neuronal differentiation; the branching of the neurites; protein synthesis; expression of neurofilament proteins present in neurites; neurotransmitter release; and the enhancement of long-term potentiation. All of these effects can be blocked by drugs that antagonize P2Y receptors. The increase in neurites among cells exposed to uridine or UTP [10
] provided the first demonstration that phosphatide precursors could alter cellular anatomy, as well as biochemical indices.
P2Y2 receptors in patients with AD apparently are deficient in the parietal cortex [13
]—a brain region important for memory. This suggests that increasing brain levels of the receptor ligand UTP—by administering the UMP in SOUVENAID®
—may partially compensate for the P2Y receptor deficiency.
Interestingly, uridine is not unique in being able to regulate cell differentiation and metabolism by two distinct mechanisms, acting both as a receptor agonist and as a bulk precursor of the CTP needed for phosphatide synthesis. Diacylglycerol also acts in two ways, both as a potent “second messenger” that activates a number of enzymes and other proteins, and also, like uridine, as a bulk precursor in phosphatide synthesis.
3.4. Incorporation of Synaptic Membrane into Dendritic Spines
The numbers of dendritic spines in particular brain regions are highly correlated with the numbers of synapses [3
], and it has been proposed that “more than 90% of excitatory synapses occur on dendritic spines” [2
]. This suggests that processes that damage the spines or, conversely, that increase spine number will cause parallel changes in synapse number. The formation of new dendritic spines in the hippocampus can be initiated by synaptic inputs that depolarize CA1 pyramidal neurons and cause long-term potentiation. This effect is probably mediated by enhanced calcium influx into the CA1 neurons.
The effects on hippocampal dendritic spine number of administering the omega-3 fatty acid DHA, alone or with UMP, were compared after one to four weeks of daily treatment [3
]. DHA alone caused dose-related increases in spine number, as well as in membrane phosphatides and synaptic proteins; its effects were doubled if animals also received UMP. Treated animals also exhibited improvements in cognitive performance as assessed using the Morris water maze test and various other tests based on traversing mazes [4
]. In contrast, administration of the omega-6 fatty acid arachidonic acid affected neither spine density nor membrane composition. Similar positive effects of giving DHA plus UMP were observed in weanlings whose mothers consumed the nutrients for 10 days prior to parturition, and for 21 days while nursing.
In summary, the simultaneous provision of DHA, uridine (as UMP) and choline increases brain phosphatides and synaptic proteins, the main constituents of synaptic membranes, as well as dendritic spines, the immediate anatomic precursor for new synapses.
Although the formation of new synapses is triggered by neuronal firing, the number of synapses that form can be modulated when three circulating nutrients, uridine, DHA, and choline are administered together. This is because cellular levels of the nutrients control the saturation of key enzymes in the synthesis of the phosphatides in synaptic membranes. One of the nutrients, uridine, also affects synaptogenesis by activating P2Y receptors in the brain.