High-density presynaptic transporters are required for glutamate removal from the first visual synapse

Reliable synaptic transmission depends not only on the release machinery and the postsynaptic response mechanism but also on removal or degradation of transmitter from the synaptic cleft. Accumulating evidence indicates that postsynaptic and glial excitatory amino acid transporters (EAATs) contribute to glutamate removal. However, the role of presynaptic EAATs is unclear. Here, we show in the mouse retina that glutamate is removed from the synaptic cleft at the rod to rod bipolar cell (RBC) synapse by presynaptic EAATs rather than by postsynaptic or glial EAATs. The RBC currents evoked by electrical stimulation of rods decayed slowly after pharmacological blockade of EAATs. Recordings of the evoked RBC currents from EAAT subtype-deficient mice and the EAAT-coupled anion current reveal that functional EAATs are localized to rod terminals. Model simulations suggest that rod EAATs are densely packed near the release site and that rods are equipped with an almost self-sufficient glutamate recollecting system.     

Biotransformation of sesquiterpenoids having α,β-unsaturated carbonyl groups with cultured plant cells of Marchantia polymorpha

The biotransformation of sesquiterpenoids having an α,β-unsaturated carbonyl group, such as α-santonin (1), lancerodiol p-hydroxybenzoate (2), 8,9-dehydronootkatone (3), and nootkatone (4), with cultured suspension cells of Marchantia polymorpha was investigated. It was found that the CC double bond of 1 and 2 was hydrogenated to give 1,2-dihydro-α-santonin (5) and 3,4-dihydrolancerodiol p-hydroxybenzoate (6), respectively, while the allylic position of the CC double bond of 3 and 4 was hydroxylated to give 13-hydroxy-8,9-dehydronootkatone (7) and 9-hydroxynootkatone (8), respectively.     

Cryoprotectant permeability of aquaporin-3 expressed in Xenopus oocytes

It has been shown that aquaporin-3, a water channel, is expressed in mouse embryos. This type of aquaporin transports not only water but also neutral solutes, including cell-permeating cryoprotectants. Therefore, the expression of this channel may have significant influence on the survival of cryopreserved embryos. However, permeability coefficients of aquaporin-3 to cryoprotectants have not been determined except for glycerol. In addition, permeability coefficients under concentration gradients are important for developing and improving cryopreservation protocols. In this study, we examined the permeability of aquaporin-3 to various cryoprotectants using Xenopus oocytes. The permeability of aquaporin-3 to cryoprotectants was measured by the volume change of aquaporin-3 cRNA-injected oocytes in modified Barth's solution containing either 10% glycerol, 8% ethylene glycol, 10% propylene glycol, 1.5 M acetamide, or 9.5% DMSO (1.51-1.83 Osm/kg) at 25 degrees C. Permeability coefficients of aquaporin-3 for ethylene glycol and propylene glycol were 33.50 and 31.45 x 10(-3) cm/min, respectively, which were as high as the value for glycerol (36.13 x 10(-3) cm/min). These values were much higher than those for water-injected control oocytes (0.04-0.11 x 10(-3) cm/min). On the other hand, the coefficients for acetamide and DMSO were not well determined because the volume data were poorly fitted by the two parameter model, possibly because of membrane damage. To avoid this, the permeability for these cryoprotectants was measured under a low concentration gradient by suspending oocytes in aqueous solutions containing low concentrations of acetamide or DMSO dissolved in water (0.20 Osm/kg). The coefficient for acetamide (24.60 x 10(-3) cm/min) was as high as the coefficients for glycerol, ethylene glycol, and propylene glycol, and was significantly higher than the value for control (6.50 x 10(-3) cm/min). The value for DMSO (6.33 x 10(-3) cm/min) was relatively low, although higher than the value for control (0.79 x 10(-3) cm/min). This is the first reported observation of DMSO transport by aquaporin-3.     

Visfatin is present in bovine mammary epithelial cells, lactating mammary gland and milk, and its expression is regulated by cAMP pathway

Visfatin was originally identified as a growth factor for immature B cells, and recently demonstrated to bind insulin receptor. Visfatin mRNA and protein were detected by RT-PCR and Western blot analysis in cloned bovine mammary epithelial cells, lactating bovine mammary gland and human breast cancer cell line, MCF-7. Immunocytochemical staining localized the visfatin protein in the cytosol and nucleus of both cells. Quantitative-RT-PCR analysis revealed that the expression of the visfatin mRNA was significantly elevated when treated with forskolin (500 microM), isopreterenol (1-10 microM) and dibutyric cyclic AMP (1 mM) for 24 h, and significantly reduced when treated with insulin (5-50 ng/ml) and dexsamethasone (0.5-250 nM) for 24 h. These results indicate that mammary epithelial cells express the visfatin protein and secrete them into the milk.     

The vascular endothelial growth factor VEGF165 induces perlecan synthesis via VEGF receptor-2 in cultured human brain microvascular endothelial cells

A member of the vascular endothelial growth factor (VEGF) family, VEGF165, regulates vascular endothelial cell functions in autocrine and paracrine fashions in microvessels. Proteoglycans are highly glycosylated poly-anionic macromolecules that influence cellular behaviors such as proliferation and migration by interacting with cytokines/growth factors. In the present study, we investigated the regulation of proteoglycan synthesis by VEGF165 in cultured human brain microvascular endothelial cells. The cells were exposed to recombinant human VEGF165, and the proteoglycans were then characterized using biochemical techniques. VEGF165 treatment increased the accumulation of proteoglycans 1.4- and 1.6-fold in the cell layer and conditioned medium, respectively. This effect resulted from the activation of VEGFR-2, and was mimicked by vammin, a VEGFR-2 ligand from snake venom but not placenta growth factor, which binds specifically to VEGFR-1. VEGF165 stimulated the production and secretion of perlecan, substituted with shorter heparan sulfate side chains, but with unaltered sulfated disaccharide composition. The perlecan secreted by VEGF165-stimulated endothelial cells may be involved in the regulation of cellular behavior during angiogenesis, in diseases of the brain microvessels, and in the maintenance of the endothelial cell monolayer.     

Assortment of phosphatidylinositol 3-kinase complexes--Atg14p directs association of complex I to the pre-autophagosomal structure in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, two similar phosphatidylinositol 3-kinase complexes (complexes I and II) function in distinct biological processes, complex I in autophagy and complex II in the vacuolar protein sorting via endosomes. Atg14p is only integrated into complex I, likely facilitating the function of complex I in autophagy. Deletion analysis of Atg14p revealed that N-terminal region containing the coiled-coil structures was essential and sufficient for autophagy. Atg14p localized to pre-autophagosomal structure (PAS) and vacuolar membranes, whereas Vps38p, a component specific to complex II, localized to endosomes and vacuolar membranes. Vps34p and Vps30p, components shared by the two complexes, localized to the PAS, vacuolar membranes, and several punctate structures that included endosomes. The localization of these components to the PAS was Atg14p dependent but not dependent on Vps38p. Conversely, localization of these proteins to endosomes required Vps38p but not Atg14p. Vps15p, regulatory subunit of the Vps34p complexes, localized to the PAS, vacuolar membranes, and punctate structures independent of both Atg14p and Vps38p. Together, these results indicate that complexes I and II function in distinct biological processes by localizing to specific compartments in a manner mediated by specific components of each complex, Atg14p and Vps38p, respectively.     

The Autophagy-related Protein Kinase Atg1 Interacts with the Ubiquitin-like Protein Atg8 via the Atg8 Family Interacting Motif to Facilitate Autophagosome Formation

In autophagy, a cup-shaped membrane called the isolation membrane is formed, expanded, and sealed to complete a double membrane-bound vesicle called the autophagosome that encapsulates cellular constituents to be transported to and degraded in the lysosome/vacuole. The formation of the autophagosome requires autophagy-related (Atg) proteins. Atg8 is a ubiquitin-like protein that localizes to the isolation membrane; a subpopulation of this protein remains inside the autophagosome and is transported to the lysosome/vacuole. In the budding yeast Saccharomyces cerevisiae, Atg1 is a serine/threonine kinase that functions in the initial step of autophagosome formation and is also efficiently transported to the vacuole via autophagy. Here, we explore the mechanism and significance of this autophagic transport of Atg1. In selective types of autophagy, receptor proteins recognize degradation targets and also interact with Atg8, via the Atg8 family interacting motif (AIM), to link the targets to the isolation membrane. We find that Atg1 contains an AIM and directly interacts with Atg8. Mutations in the AIM disrupt this interaction and abolish vacuolar transport of Atg1. These results suggest that Atg1 associates with the isolation membrane by binding to Atg8, resulting in its incorporation into the autophagosome. We also show that mutations in the Atg1 AIM cause a significant defect in autophagy, without affecting the functions of Atg1 implicated in triggering autophagosome formation. We propose that in addition to its essential function in the initial stage, Atg1 also associates with the isolation membrane to promote its maturation into the autophagosome.     

Membrane Protein Rim21 Plays a Central Role in Sensing Ambient pH in Saccharomyces cerevisiae

External alkalization activates the Rim101 pathway in Saccharomyces cerevisiae. In this pathway, three integral membrane proteins, Rim21, Dfg16, and Rim9, are considered to be the components of the pH sensor machinery. However, how these proteins are involved in pH sensing is totally unknown. In this work, we investigated the localization, physical interaction, and interrelationship of Rim21, Dfg16, and Rim9. These proteins were found to form a complex and to localize to the plasma membrane in a patchy and mutually dependent manner. Their cellular level was also mutually dependent. In particular, the Rim21 level was significantly decreased in dfg16Δ and rim9Δ cells. Upon external alkalization, the proteins were internalized and degraded. We also demonstrate that the transient degradation of Rim21 completely suppressed the Rim101 pathway but that the degradation of Dfg16 or Rim9 did not. This finding strongly suggests that Rim21 is the pH sensor protein and that Dfg16 and Rim9 play auxiliary functions through maintaining the level of Rim21 and assisting in its plasma membrane localization. Even without external alkalization, the Rim101 pathway was activated in a Rim21-dependent manner by either protonophore treatment or depletion of phosphatidylserine in the inner leaflet of the plasma membrane, both of which caused plasma membrane depolarization like the external alkalization. Therefore, plasma membrane depolarization seems to be one of the key signals for the pH sensor molecule Rim21.     

O-linked-N-acetylglucosamine modification of mammalian Notch receptors by an atypical O-GlcNAc transferase Eogt1

O-linked-β-N-acetylglucosamine (O-GlcNAc) modification is a unique cytoplasmic and nuclear protein modification that is common in nearly all eukaryotes, including filamentous fungi, plants, and animals. We had recently reported that epidermal growth factor (EGF) repeats of Notch and Dumpy are O-GlcNAcylated by an atypical O-GlcNAc transferase, EOGT, in Drosophila. However, no study has yet shown whether O-GlcNAcylation of extracellular proteins is limited to insects such as Drosophila or whether it occurs in other organisms, including mammals. Here, we report the characterization of A130022J15Rik, a mouse gene homolog of Drosophila Eogt (Eogt 1). Enzymatic analysis revealed that Eogt1 has a substrate specificity similar to that of Drosophila EOGT, wherein the Thr residue located between the fifth and sixth conserved cysteines of the folded EGF-like domains is modified. This observation is supported by the fact that the expression of Eogt1 in Drosophila rescued the cell-adhesion defect caused by Eogt downregulation. In HEK293T cells, Eogt1 expression promoted modification of Notch1 EGF repeats by O-GlcNAc, which was further modified, at least in part, by galactose to generate a novel O-linked-N-acetyllactosamine structure. These results suggest that Eogt1 encodes EGF domain O-GlcNAc transferase and that O-GlcNAcylation reaction in the secretory pathway is a fundamental biochemical process conserved through evolution.