Choline is an essential nutrient for all cells because it plays a role in the synthesis of the membrane phospholipid components of the cell membranes, as a methyl-group donor in methionine metabolism as well as in the synthesis of the neurotransmitter acetylcholine. Choline deficiency affects the expression of genes involved in cell proliferation, differentiation, and apoptosis, and it has been associated with liver dysfunction and cancer. Abnormal choline transport and metabolism have been implicated in a number of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therefore, the study of choline transport and the characteristics of choline transporters are of central importance to understanding the mechanisms that underlie membrane integrity and cell signaling in such disorders. Kinetic studies with radiolabeled choline and inhibitors distinguish three systems for choline transport: (i) low-affinity facilitated diffusion, (ii) high-affinity, Na+-dependent transport, and (iii) intermediate-affinity, Na+-independent transport. It is only recently, however, that the proteins having transport characteristics of at least one of these systems have been identified. They include (i) polyspecific organic cation transporters (OCTs) with low affinity for choline, (ii) high-affinity choline transporters (CHT1s), and (iii) intermediate-affinity choline transporter-like (CTL1) proteins. CHT1 and CTL1 but not OCT transporters are selectively inhibited with hemicholinium-3 and essentially display characteristics of specialized transporters for targeted choline metabolism. CHT1 is abundant in neurons and almost exclusively supplies choline for acetyl-choline synthesis. The focus here is more on newly-discovered CTL1 choline transporters. They are expressed in different organisms and cell types, apparently not for the biosynthesis of acetylcholine but for the production of the most abundant metabolite of choline, the membrane lipid phosphatidylcholine.

In a recent study, we reported that in bovine brain extract, glycogen synthase kinase-3␤ and tau are parts of an ϳ400 -500 kDa microtubule-associated tau phosphorylation complex (Sun, W., Qureshi, H. Y., Cafferty, P. W., Sobue, K., Agarwal-Mawal, A., Neufield, K. D., and Paudel, H. K. (2002) J. Biol. Chem. 277, 11933-11940). In this study, we find that when purified brain microtubules are subjected to Superose 12 gel filtration column chromatography, the dimeric scaffold protein 14-3-3 coelutes with the tau phosphorylation complex components tau and GSK3␤. From gel filtration fractions containing the tau phosphorylation complex, 14-3-3, GSK3␤, and tau co-immunoprecipitate with each other. From extracts of bovine brain, COS-7 cells, and HEK-293 cells transfected with GSK3␤, 14-3-3 co-precipitates with GSK3␤, indicating that GSK3␤ binds to 14-3-3. From HEK-293 cells transfected with tau, GSK3␤, and 14-3-3 in different combinations, tau co-immunoprecipitates with GSK3␤ only in the presence of 14-3-3. In vitro, ϳ10-fold more tau binds to GSK3␤ in the presence of than in the absence of 14-3-3. In transfected HEK-293 cells, 14-3-3 stimulates GSK3␤-catalyzed tau phosphorylation in a dose-dependent manner. These data indicate that in brain, the 14-3-3 dimer simultaneously binds and bridges tau and GSK3␤ and stimulates GSK3␤-catalyzed tau phosphorylation.

Adiponectin, a protein secreted from adipose tissue, has been shown to have anti-diabetic and anti-inflammatory effects, but its regulation is not completely understood. Long-chain n-3 fatty acids eicosapentaenoic acid (20:5n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) may be involved in adiponectin regulation as they are potential ligands for peroxisome proliferator-activated receptor-γ (PPARγ), a key transcription factor for the adiponectin gene. To examine this, 3T3-L1 adipocytes were incubated with 125 µmol·L-1 EPA, DHA, palmitic, or oleic acids complexed to albumin, or with albumin alone (control) for 24 h. Adipocytes were also incubated for 24 h with EPA and DHA plus bisphenol-A-diglycidyl ether (BADGE), a PPARγ antagonist. Both EPA and DHA increased (p < 0.05) secreted adiponectin concentration compared with the control (44% and 102%, respectively), but did not affect cellular adiponectin protein content. Incubation with BADGE and DHA inhibited increases in secreted adiponectin protein, suggesting that DHA may act through a PPARγ-dependent mechanism. However, BADGE had no effect on EPA-induced increases in secreted adiponectin protein. Only DHA enhanced (p < 0.05) PPARγ and adiponectin mRNA expression compared wtih the control. Our results demonstrate that DHA increases cellular adiponectin mRNA and secreted adiponectin protein in 3T3-L1 adipocytes, possibly by a mechanism involving PPARγ. Moreover, DHA increased adiponectin concentration to a greater extent (40% more, p < 0.05) compared with EPA, emphasizing the need to consider the independent actions of EPA and DHA in adipocytes.

Circulating microRNAs, present either in the cellular component, peripheral blood mononuclear cells (PBMC), or in cell-free plasma, have emerged as biomarkers for age-dependent systemic, disease-associated changes in many organs. Previously, we have shown that microRNA (miR)-34a is increased in circulating PBMC of Alzheimer's disease (AD) patients. In the present study, we show that this microRNA's sister, miR-34c, exhibits even greater increase in both cellular and plasma components of AD circulating blood samples, compared to normal age-matched controls. Statistical analysis shows the accuracy of levels of miR-34c assayed by receiver operating characteristic (ROC) analysis: the area under the curve is 0.99 (p < 0.0001) and the 95% confidence level extends from 0.97 to 1. Pearson correlation between miR-34c levels and mild and moderate AD, as defined by the mini-mental state examination (MMSE), shows an r-value of −0.7, suggesting a relatively strong inverse relationship between the two parameters. These data show that plasma levels of microRNA 34c are much more prominent in AD than those of its sister, miR-34a, or than its own level in PBMC. Transfection studies show that miR-34c, as does its sister miR-34a, represses the expression of several selected genes involved in cell survival and oxidative defense pathways, such as Bcl2, SIRT1, and others, in cultured cells. Taken together, our results indicate that increased levels of miR-34c in both PBMC and plasma may reflect changes in circulating blood samples in AD patients, compared to age-matched normal controls.

In mammalian brain, tau, glycogen synthase kinase 3␤ (GSK3␤), and 14-3-3, a phosphoserine-binding protein, are parts of a multiprotein tau phosphorylation complex. Within the complex, 14-3-3 simultaneously binds to tau and GSK3␤ (Agarwal-Mawal, A., Qureshi, H. Y., Cafferty, P. W., Yuan, Z., Han, D., Lin, R., and Paudel, H. K. (2003) J. Biol. Chem. 278, 12722-12728). The molecular mechanism by which 14-3-3 connects GSK3␤ to tau within the complex is not clear. In this study, we find that GSK3␤ within the tau phosphorylation complex is phosphorylated on Ser 9 . From extracts of rat brain and rat primary cultured neurons, Ser 9 -phosphorylated GSK3␤ precipitates with glutathione-agarose beads coated with glutathione S-transferase-14-3-3. Similarly, from rat brain extract, Ser 9 -phosphorylated GSK3␤ coimmunoprecipitates with tau. In vitro, 14-3-3 binds to GSK3␤ only when the kinase is phosphorylated on Ser 9 . In transfected HEK-293 cells, 14-3-3 binds to Ser 9 -phosphorylated GSK3␤ and does not bind to GSK3␤ (S9A). Tau, on the other hand, binds to both GSK3␤ (WT) and GSK3␤ (S9A). Moreover, 14-3-3 enhances the binding of tau with Ser 9 -phosphorylated GSK3␤ by ϳ3-fold but not with GSK3␤ (S9A). Similarly, 14-3-3 stimulates phosphorylation of tau by Ser 9 -phosphorylated GSK3␤ but not by GSK3␤ (S9A). In transfected HEK-293 cells, Ser 9 phosphorylation suppresses GSK3␤-catalyzed tau phosphorylation in the absence of 14-3-3. In the presence of 14-3-3, however, Ser 9 -phosphorylated GSK3␤ remains active and phosphorylates tau. Our data indicate that within the tau phosphorylation complex, 14-3-3 connects Ser 9 -phosphorylated GSK3␤ to tau and Ser 9 -phosphorylated GSK3␤ phosphorylates tau.

Despite decades of studying muscle glycogen in many metabolic situations, surprisingly little is known regarding its regulation. Glycogen is a dynamic and vital metabolic fuel that has very limited energetic capacity. Thus its regulation is highly complex and multifaceted. The stores in muscle are not homogeneous and there appear to be various metabolic pools. Each granule is capable of independent regulation and fundamental aspects of the regulation appear to be associated with a complex set of proteins (some are enzymes and others serve scaffolding roles) that associate both with the granule and with each other in a dynamic fashion. The regulation includes altered phosphorylation status and often translocation as well. The understanding of the roles and the regulation of glycogenin, protein phosphatase 1, glycogen targeting proteins, laforin and malin are in their infancy. These various processes appear to be the mechanisms that give the glycogen granule precise, yet dynamic regulation.

No abstract

The present study investigates choline transport processes and regulation of choline transporter-like protein-1 (CTL1) in human THP-1 monocytic cells and phorbol myristate 13-acetate (PMA)-differentiated macrophages. Choline uptake is saturable and therefore protein-mediated in both cell types, but its transport characteristics change soon after treatments with PMA. The maximal rate of choline uptake intrinsic to monocytic cells is greatly diminished in differentiated macrophages as demonstrated by alterations in V(max) values from 1,973 +/- 118 to 380 +/- 18 nmol x mg(-1) x min(-1), when the binding affinity did not change significantly (K(m) values 56 +/- 8 and 53 +/- 6 microM, respectively). Treatments with hemicholinim-3 effectively inhibit most of the choline uptake, establishing that a choline-specific transport protein rather than a general transporter is responsible for the observed kinetic parameters. mRNA screening for the expression of various transporters reveals that CTL1 is the most plausible candidate that possesses the described kinetic and inhibitory properties. Fluorescence-activated cell sorting analyses at various times after PMA treatments further demonstrate that the disappearance of CTL1 protein from the cell surface follows the same trend as the reduction in choline uptake. Importantly, the loss of functional CTL1 from the cell surface occurs without significant changes in total CTL1 protein or its mRNA level indicating that an impaired CTL1 trafficking is the key contributing factor to the reduced choline uptake, subsequent to the PMA-induced THP-1 differentiation to macrophages.