Possible Contribution of Taurine to Distorted Glucagon Secretion in Intra-Islet Insulin Deficiency: A Metabolome Analysis Using a Novel α-Cell Model of Insulin-Deficient Diabetes

Glycemic instability is a serious problem in patients with insulin-deficient diabetes, and it may be due in part to abnormal endogenous glucagon secretion. However, the intracellular metabolic mechanism(s) involved in the aberrant glucagon response under the condition of insulin deficiency has not yet been elucidated. To investigate the metabolic traits that underlie the distortion of glucagon secretion under insulin deficient conditions, we generated an αTC1-6 cell line with stable knockdown of the insulin receptor (IRKD), i.e., an in vitro α-cell model for insulin-deficient diabetes, which exhibits an abnormal glucagon response to glucose. A comprehensive metabolomic analysis of the IRKD αTC1-6 cells (IRKD cells) revealed some candidate metabolites whose levels differed markedly compared to those in control αTC1-6 cells, but also which could affect the glucagon release in IRKD cells. Of these candidates, taurine was remarkably increased in the IRKD cells and was identified as a stimulator of glucagon in αTC1-6 cells. Taurine also paradoxically exaggerated the glucagon secretion at a high glucose concentration in IRKD cells and islets with IRKD. These results indicate that the metabolic alterations induced by IRKD in α-cells, especially the increase of taurine, may lead to the distorted glucagon response in IRKD cells, suggesting the importance of taurine in the paradoxical glucagon response and the resultant glucose instability in insulin-deficient diabetes.     

Preparation and diastereomeric separation of an (S)- and (R)-1-(methoxycarbonyl)tridec-10-yl glucoside derivative,a precursor for a monosaccharide constituent of resin glycosides

The 6-O-mesyl derivative of phenyl 1-thio-beta-D-glucopyranoside was prepared from D-glucose as a synthetic equivalent of a 6-deoxy-hexosyl donor. Racemic methyl 11-hydroxytetradecanoate (methyl convolvulinolate) was synthesized by Grignard reaction of propylmagnesium bromide with 10-undecenal followed by hydroboration. Both intermediates were coupled by NIS-TfOH-promoted glycosidation to give a mixture of two diasteromeric glucopyranosides, which were separated on a preparative scale by medium pressure chromatography. One of the products was identified as having the natural (S)-configuration by comparison of its 1H NMR spectrum with an authentic sample prepared from the corresponding chiral hydroxyfatty acid.     

Effect of synthetic 1-34 fragment of human parathyroid hormone on plasma adenosine 3',5'-monophosphate (cAMP) concentrations and the diagnostic criteria based on the plasma cAMP response in Ellsworth-Howard test

The effect of synthetic 1-34 fragment of human parathyroid hormone (hPTH(1-34] on plasma adenosine 3',5'-monophosphate (cAMP) in human subjects and the diagnostic criteria for the plasma cAMP response in an Ellsworth-Howard test were studied. 20 or 30 micrograms hPTH(1-34) and 200 USP Parathormone (Eli Lilly & Co.), infused intravenously over 5 min, produced very similar patterns of response in plasma cAMP, peak values being observed within 5 or 10 min after the end of the infusion. The maximum levels of plasma cAMP were over 111.5 pmol/ml in all of the normal subjects (n = 5) and patients with idiopathic hypoparathyroidism (n = 22), including those of children, but the plasma cAMP did not rise above 65.0 pmol/ml in pseudohypoparathyroidism (n = 7). There existed a significant correlation between the maximum plasma cAMP concentrations and increases in urinary cAMP excretion after infusions of both hPTH(1-34) and Parathormone. These results suggest that hPTH(1-34) has effects essentially identical to those of native PTH on plasma cAMP. We would like to propose a new diagnostic criterion in the Ellsworth-Howard test: a peak value of plasma cAMP over 100 pmol/ml after 30 micrograms hPTH(1-34) infusion is regarded as a normal response.     

Assay of centromere function using a human artificial chromosome

In order to define a functional human centromere sequence, an artificial chromosome was constructed as a reproducible DNA molecule. Mammalian telomere repeats and a selectable marker were introduced into yeast artificial chromosomes (YACs) containing alphoid DNA from the centromere region of human chromosome 21 in a recombination-deficient yeast host. When these modified YACs were introduced into cultured human cells, a YAC with the alphoid DNA from the α21-I locus, containing CENP-B boxes at a high frequency and a regular repeat array, efficiently formed minichromosomes that were maintained stably in the absence of selection and bound CENP-A, CENP-B, CENP-C and CENP-E. The minichromosomes, 1–5 Mb in size and composed of multimers of the introduced YAC DNA, aligned at metaphase plates and segregated to opposite poles correctly in anaphase. Extensive cytological analyses strongly suggested that the minichromosomes had not acquired host sequences and were formed in all cases by a de novo mechanism. In contrast, minichromosomes were never produced with a modified YAC containing alphoid DNA from the α21-II locus, which contains no CENP-B boxes and has a less regular sequence arrangement. We conclude that α21-I alphoid DNA can induce de novo assembly of active centromere/kinetochore structures on minichromosomes. Received: 22 August 1998 / Accepted: 28 August 1998     

Channel-dependent permeation of water and glycerol in mouse morulae

The cryosensitivity of mammalian embryos depends on the stage of development. Because permeability to water and cryoprotectants plays an important role in cryopreservation, it is plausible that the permeability is involved in the difference in the tolerance to cryopreservation among embryos at different developmental stages. In this study, we examined the permeability to water and glycerol of mouse oocytes and embryos, and tried to deduce the pathway for the movement of water and glycerol. The water permeability (L(P), microm min(-1) atm(-1)) of oocytes and four-cell embryos at 25 degrees C was low (0.63-0.70) and its Arrhenius activation energy (E(a), kcal/mol) was high (11.6-12.3), which implies that the water permeates through the plasma membrane by simple diffusion. On the other hand, the L(p) of morulae and blastocysts was quite high (3.6-4.5) and its E(a) was quite low (5.1-6.3), which implies that the water moves through water channels. Aquaporin inhibitors, phloretin and p-(chloromercuri) benzene-sulfonate, reduced the L(p) of morulae significantly but not that of oocytes. By immunocytochemical analysis, aquaporin 3, which transports not only water but also glycerol, was detected in the morulae but not in the oocytes. Accordingly, the glycerol permeability (P(GLY), x 10(-3) cm/min) of oocytes was also low (0.01) and its E(a) was remarkably high (41.6), whereas P(GLY) of morulae was quite high (4.63) and its E(a) was low (10.0). Aquaporin inhibitors reduced the P(GLY) of morulae significantly. In conclusion, water and glycerol appear to move across the plasma membrane mainly by simple diffusion in oocytes but by facilitated diffusion through water channel(s) including aquaporin 3 in morulae.     

Mechanism for manganese enhancement of dopamine-induced oxidative DNA damage and neuronal cell death

Although the cause of dopaminergic cell death in Parkinson's disease is still poorly understood, there is accumulating evidence suggesting that metal ions can be involved in the processes. We investigated the effect of manganese on cell death and DNA damage in PC12 cells treated with dopamine. Mn(II) enhanced cell death induced by dopamine. Mn(II) also increased the 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) contents of DNA in PC12 cells treated with dopamine. To clarify the mechanism of cellular DNA damage, we investigated DNA damage induced by dopamine and Mn(II) using (32)P-labeled DNA fragments. Mn(II) enhanced Cu(II)-dependent DNA damage by dopamine. The Mn(II)-enhanced DNA damage was greatly increased by NADH. Piperidine and formamidopyrimidine-DNA glycosylase treatment induced cleavage sites mainly at T and G of the 5'-TG-3' sequence, respectively. Bathocuproine, a Cu(I) chelator, and catalase inhibited the DNA damage. Oxygen consumption and UV-visible spectroscopic measurements showed that Mn(II) enhanced autoxidation of dopamine with H(2)O(2) formation. These results suggest that reactive species derived from the reaction of H(2)O(2) with Cu(I) participates in Mn(II)-enhanced DNA damage by dopamine plus Cu(II). Therefore, it is concluded that oxidative DNA damage induced by dopamine in the presence of Mn(II), NADH, and Cu(II) is possibly linked to the degeneration of dopaminergic neurons.     

Structure of the zinc‐bound amino‐terminal domain of the NMDA receptor NR2B subunit

N‐methyl‐D ‐aspartate (NMDA) receptors belong to the family of ionotropic glutamate receptors (iGluRs) that mediate the majority of fast excitatory synaptic transmission in the mammalian brain. One of the hallmarks for the function of NMDA receptors is that their ion channel activity is allosterically regulated by binding of modulator compounds to the extracellular amino‐terminal domain (ATD) distinct from the L ‐glutamate‐binding domain. The molecular basis for the ATD‐mediated allosteric regulation has been enigmatic because of a complete lack of structural information on NMDA receptor ATDs. Here, we report the crystal structures of ATD from the NR2B NMDA receptor subunit in the zinc‐free and zinc‐bound states. The structures reveal the overall clamshell‐like architecture distinct from the non‐NMDA receptor ATDs and molecular determinants for the zinc‐binding site, ion‐binding sites, and the architecture of the putative phenylethanolamine‐binding site.