Elsevier

Brain Research

Volume 1088, Issue 1, 9 May 2006, Pages 83-92
Brain Research

Research Report
Synaptic proteins and phospholipids are increased in gerbil brain by administering uridine plus docosahexaenoic acid orally

https://doi.org/10.1016/j.brainres.2006.03.019Get rights and content

Abstract

The synthesis of brain phosphatidylcholine may utilize three circulating precursors: choline; a pyrimidine (e.g., uridine, converted via UTP to brain CTP); and a PUFA (e.g., docosahexaenoic acid); phosphatidylethanolamine may utilize two of these, a pyrimidine and a PUFA. We observe that consuming these precursors can substantially increase membrane phosphatide and synaptic protein levels in gerbil brains. (Pyrimidine metabolism in gerbils, but not rats, resembles that in humans.) Animals received, daily for 4 weeks, a diet containing choline chloride and UMP (a uridine source) and/or DHA by gavage. Brain phosphatidylcholine rose by 13–22% with uridine and choline alone, or DHA alone, or by 45% with the combination, phosphatidylethanolamine and the other phosphatides increasing by 39–74%. Smaller elevations occurred after 1–3 weeks. The combination also increased the vesicular protein Synapsin-1 by 41%, the postsynaptic protein PSD-95 by 38% and the neurite neurofibrillar proteins NF-70 and NF-M by up to 102% and 48%, respectively. However, it had no effect on the cytoskeletal protein beta-tubulin. Hence, the quantity of synaptic membrane probably increased. The precursors act by enhancing the substrate saturation of enzymes that initiate their incorporation into phosphatidylcholine and phosphatidylethanolamine and by UTP-mediated activation of P2Y receptors. Alzheimer's disease brains contain fewer and smaller synapses and reduced levels of synaptic proteins, membrane phosphatides, choline and DHA. The three phosphatide precursors might thus be useful in treating this disease.

Introduction

The synthesis in brain of phosphatidylcholine (PtdCho) and other membrane phosphatides can utilize, besides glucose, three compounds obtained from the circulation (Kennedy and Weiss, 1956): choline; a pyrimidine like uridine; and a polyunsaturated fatty acid (PUFA) like docosahexaenoic acid (DHA) (Rapoport, 2001, Marszalek and Lodish, 2005); all three readily cross the blood–brain barrier (Cornford et al., 1978, Li et al., 2001, Spector, 2001, Hashimoto et al., 2002). The choline is phosphorylated to form phosphocholine through the action of choline kinase (CK), a low-affinity enzyme that is unsaturated with choline at normal brain choline levels (Spanner and Ansell, 1979, Millington and Wurtman, 1982). The uridine is phosphorylated by uridine–cytidine kinase (Suzuki et al., 2004) to uridine triphosphate (UTP), which is further transformed by the enzyme CTP synthetase (Genchev and Mandel, 1974) to cytidine triphosphate (CTP), the rate-limiting precursor in PtdCho synthesis (Ross et al., 1997). Both of these latter enzymes are also low affinity, hence giving a single oral dose of uridine-5′-monophosphate (UMP), a uridine source sequentially increases brain uridine, UTP and CTP (Cansev et al., 2005). The phosphocholine and CTP combine to form cytidine-5′-diphosphocholine (CDP-choline), which then combines with diacylglycerol (DAG), including species containing DHA or another PUFA, to yield the PtdCho. The Kennedy cycle similarly synthesizes phosphatidylethanolamine (PtdEtn) from uridine and PUFA like DHA, but starting with ethanolamine, instead of choline.

Although DHA is found in PtdCho, PtdEtn and other brain membrane phosphatides (Marszalek and Lodish, 2005, Knapp and Wurtman, 1999), apparently no information is available concerning the effects of DHA's oral administration on brain phosphatide levels in vivo. Moreover, while a single dose of UMP has been shown to increase brain CDP-choline levels (Cansev et al., 2005), suggesting that it also accelerates PtdCho synthesis (Lopez-Coviella et al., 1995), no direct evidence is available that any treatment regimen involving uridine also affects brain PtdCho levels. We now show that oral administration of DHA or UMP, given alone for several weeks to animals consuming a choline-containing diet, can increase brain PtdCho and other major membrane phosphatides. Moreover, the effect of giving both DHA and UMP tends to be greater (P < 0.01 for SM) than the sum of the effects observed when each is given separately. This increase may include synaptic membranes inasmuch as the treatment also increases levels of presynaptic and postsynaptic proteins.

Section snippets

Effects of UMP and/or DHA on brain phosphatides

Animals received just the control diet or one of the three experimental treatments for 4 weeks; brain samples were assayed as described in Experimental Procedures, and phosphatide levels were given as nmol/mg protein. Addition of UMP to the standard diet without concurrent DHA treatment significantly increased brain levels of PtdCho, PtdEtn and PtdIns by 13%, 29% and 48%, respectively (Table 1A). Administration of DHA, without UMP, also significantly increased brain levels of these phosphatides

Discussion

These data show that administering DHA by gavage or UMP via the diet, daily for 4 weeks, significantly increases brain membrane PtdCho levels (Table 1A) among gerbils consuming a standard, choline-containing diet. These effects are observed whether the PtdCho is expressed per mg protein or per cell (DNA), indicating that each brain cell, on average, contains more of the membrane phosphatide. Moreover, when all three of the potentially limiting circulating precursors (i.e., choline, uridine and

Drugs and chemicals

DHA was purchased from Nu-Chek Prep, Inc. (Elysian, MN, USA). UMP was kindly provided by Numico Research (Wageningen, Netherlands). Control and UMP-containing diets were prepared by Harlan-Teklad (Madison, WI, USA). Standards for phospholipids were purchased from Sigma Chemicals (St. Louis, MO, USA). Mouse anti-NF-70, mouse anti-tubulin beta III subunit, and rabbit anti-NF-M were purchased from Chemicon (Temecula, CA, USA); mouse anti-PSD-95 from Upstate (Lake Placid, NY, USA); and mouse

Acknowledgments

The authors thank Dr. Mark Vangel for his help with statistical analyses. This work was supported by grants from the National Institutions of Health (Grant MH-28783), the Center for Brain Sciences and Metabolism Charitable Trust and the Turkish Academy of Sciences (IH Ulus).

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