Examination of the rate of peptide biosynthesis in neuroendocrine cell lines using a stable isotopic label and mass spectrometry

The biosynthesis of neuroendocrine peptides is typically examined by following the rate of appearance of a radioactive amino acid into mature forms of peptides. In the present study, we labeled cell lines with L-leucine containing 10 deuterium residues (d(10)-Leu) and used mass spectrometry to measure the biosynthetic rate of gamma-lipotropin in the AtT-20 cell line and insulin in the INS-1 cell line. After 3 h of labeling, both peptides show detectable levels of the d-labeled form in the cells and media. The relative levels of the d-labeled forms are greater in the media than in the cells, consistent with previous studies that found that newly synthesized peptides are secreted at a higher rate than older peptides under basal conditions. When AtT-20 cells were stimulated with KCl or forskolin, the ratio of d- to H-labeled gamma-lipotropin in the medium decreased, suggesting that the older peptide was in a compartment that could be released upon the appropriate stimulation. Overexpression of proSAAS in AtT-20 cells reduced the ratio of d- to H-labeled gamma-lipotropin, consistent with the proposed role of proSAAS as an endogenous inhibitor of prohormone convertase-1. Labeling with d10-Leu was also used to test whether altering the pH of the secretory pathway with chloroquine affected the rate of peptide biosynthesis. In AtT-20 cells, 30 microm chloroquine for 3 or 6 h significantly reduced the rate of formation of gamma-lipotropin in both cells and media. Similarly, INS-1 cells treated with 10, 30, or 60 microm chloroquine for 6 h showed a significant decrease in the rate of formation of insulin in both cells and media. These results are consistent with the acidic pH optima for peptide processing enzymes. Stable isotopic labeling with d10-Leu provides a sensitive method to examine the rate of peptide formation in neuroendocrine cell lines.     

人胰岛素原C肽固相合成的改进

利用谷氨酸的γ羧基与 4 甲基二苯甲基胺 (MBHA)树脂的氨基偶联固相 ,合成经HF切割去保护基后形成C端是谷氨酰胺的人胰岛素原C肽。MBHA树脂相对价格便宜 ,此方法是制备人胰岛素原C肽的另一种尝试。同时 ,也用PAM树脂合成了人胰岛素原C肽类似物。     

Cathepsin L participates in the production of neuropeptide Y in secretory vesicles, demonstrated by protease gene knockout and expression

Neuropeptide Y (NPY) functions as a peptide neurotransmitter and as a neuroendocrine hormone. The active NPY peptide is generated in secretory vesicles by proteolytic processing of proNPY. Novel findings from this study show that cathepsin L participates as a key proteolytic enzyme for NPY production in secretory vesicles. Notably, NPY levels in cathepsin L knockout (KO) mice were substantially reduced in brain and adrenal medulla by 80% and 90%, respectively. Participation of cathepsin L in producing NPY predicts their colocalization in secretory vesicles, a primary site of NPY production. Indeed, cathepsin L was colocalized with NPY in brain cortical neurons and in chromaffin cells of adrenal medulla, demonstrated by immunofluorescence confocal microscopy. Immunoelectron microscopy confirmed the localization of cathepsin L with NPY in regulated secretory vesicles of chromaffin cells. Functional studies showed that coexpression of proNPY with cathepsin L in neuroendocrine PC12 cells resulted in increased production of NPY. Furthermore , in vitro processing indicated cathepsin L processing of proNPY at paired basic residues. These findings demonstrate a role for cathepsin L in the production of NPY from its proNPY precursor. These studies illustrate the novel biological role of cathepsin L in the production of NPY, a peptide neurotransmitter, and neuroendocrine hormone.     

Human proinsulin C-peptide from a precursor overexpressed in Pichia pastoris

In this article we report the production of human proinsulin C-peptide with 31 amino acid residues from a precursor overexpressed in Pichia pastoris. A C-peptide precursor expression plasmid containing nine C-peptide genes in tandem was constructed and used to transform P. pastoris. Transformants with a high copy number of the C-peptide precursor gene integrated into the chromosome of P. pastoris were selected. In high-density fermentation in a 300 liter fermentor using a simple culture medium composed mainly of salt and methanol, the C-peptide precursor was overexpressed to a level of 2.28 g per liter. A simple procedure was established to purify the expression product from the culture medium. The purified C-peptide precursor was converted into C-peptide by trypsin and carboxypeptidase B joint digestion. The yield of C-peptide with a purity of 96% was 730 mg per liter of culture. The purified C-peptide was characterized by mass spectrometry, N- and C-terminal amino acid sequencing, and sodium dodecylsulfate-polyacrylamide gel electrophoresis. Key words proinsulin; C-peptide; Pichia pastoris     

用改进的寡核苷酸诱导突变法改造人胰岛素前体基因

我们用改进了的寡聚核苷酸诱导突变法,将两个单一限制性内切酶位点引入人胰岛素前体B链与C链连接处附近,及C链与A铁连接处附近。用含U模板法将B链第30个残基密码子ACC改成ACG,从而引入MluⅠ位点。用修改了的缺口双链DNA突变法,将A链第4个残基密码于GAG改成CTG,引入了一个PstⅠ位点。突变效率的为17％-36％。这个突变体在人胰岛素前体结构及蛋白质折叠的研究中,将有利于更换不同的C-肽。     

Muted蛋白介导CD63在嗜铬细胞大致密核粒的定位

大致密核心颗粒(Large dense-core vesicles,LDCVs)是一种溶酶体相关细胞器(Lysosome-related organelles,LROs),在细胞受到刺激时快速释放其内含物,从而调节机体生长发育、物质代谢和能量代谢等,维持机体的稳态。Muted蛋白是溶酶体相关细胞器生物发生复合体-1(Biogenesis of lysosomal organelles complex-1,BLOC-1)的一个亚基,参与调控溶酶体和多种细胞特异性LROs的生物学发生。四联体跨膜蛋白CD63最初被定位在内体-溶酶体系统,后来发现它也参与部分LROs膜的组成。CD63是否存在于LDCVs尚不清楚,其靶向运输过程是否依赖Muted蛋白也不明确。本研究以肾上腺嗜铬细胞为细胞模型,采用荧光共定位、活细胞追踪和密度梯度离心等实验鉴定CD63蛋白为LDCVs的膜组分,并探讨了其生物学功能。活细胞实验显示CD63-YFP特异性定位在NPY-dsRed标记的LDCVs上,并动态参与LDCVs膜的组成;密度梯度离心实验表明高密度区的CD63与LDCVs的标记蛋白VMAT1共同出峰;Muted蛋白缺乏的小鼠(Bloc1s5基因突变)是一种理想的Hermansky-Pudlak综合征(HPS)小鼠模型, 免疫印迹实验显示该突变体小鼠肾上腺组织中CD63蛋白含量明显减少,暗示Muted蛋白可能参与CD63的分选。以上结果表明CD63是LDCVs的膜成分,CD63在胞内的稳态水平依赖于Muted蛋白,为HPS的病理发生机制提供一定的理论依据。     

A rapid exocytosis mode in chromaffin cells with a neuronal phenotype

We have used astrocyte-conditioned medium (ACM) to promote the transdifferentiation of bovine chromaffin cells and study modifications in the exocytotic process when these cells acquire a neuronal phenotype. In the ACM-promoted neuronal phenotype, secretory vesicles and intracellular Ca2+ rise were preferentially distributed in the neurite terminals. Using amperometry, we observed that the exocytotic events also occurred mainly in the neurite terminals, wherein the individual exocytotic events had smaller quantal size than in undifferentiated cells. Additionally, duration of pre-spike current was significantly shorter, suggesting that ACM also modifies the fusion pore stability. After long exposure (7-9 days) to ACM, the kinetics of catecholamine release from individual vesicles was markedly accelerated. The morphometric analysis of vesicle diameters suggests that the rapid exocytotic events observed in neurites of ACM-treated cells correspond to the exocytosis of large dense-core vesicles (LDCV). On the other hand, experiments performed in EGTA-loaded cells suggest that ACM treatment promotes a better coupling between voltage-gated calcium channels (VGCC) and LDCV. Thus, our findings reveal that ACM promotes a neuronal phenotype in chromaffin cells, wherein the exocytotic kinetics is accelerated. Such rapid exocytosis mode could be caused at least in part by a better coupling between secretory vesicles and VGCC.     

Enrichment of Presenilin 1 Peptides in Neuronal Large Dense-Core and Somatodendritic Clathrin-Coated Vesicles

Abstract: Presenilin 1 is an integral membrane protein specifically cleaved to yield an N-terminal and a C-terminal fragment, both membrane-associated. More than 40 presenilin 1 mutations have been linked to early-onset familial Alzheimer disease, although the mechanism by which these mutations induce the Alzheimer disease neuropathology is not clear. Presenilin 1 is expressed predominantly in neurons, suggesting that the familial Alzheimer disease mutants may compromise or change the neuronal function(s) of the wild-type protein. To elucidate the function of this protein, we studied its expression in neuronal vesicular systems using as models the chromaffin granules of the neuroendocrine chromaffin cells and the major categories of brain neuronal vesicles, including the small clear-core synaptic vesicles, the large dense-core vesicles, and the somatodendritic and nerve terminal clathrin-coated vesicles. Both the N- and C-terminal presenilin 1 proteolytic fragments were greatly enriched in chromaffin granule and neuronal large dense-core vesicle membranes, indicating that these fragments are targeted to these vesicles and may regulate the large dense-core vesicle-mediated secretion of neuropeptides and neurotransmitters at synaptic sites. The presenilin 1 fragments were also enriched in the somatodendritic clathrin-coated vesicle membranes, suggesting that they are targeted to the somatodendritic membrane, where they may regulate constitutive secretion and endocytosis. In contrast, these fragments were not enriched in the small clear-core synaptic vesicle or in the nerve terminal clathrin-coated vesicle membranes. Taken together, our data indicate that presenilin 1 proteolytic fragments are targeted to specific populations of neuronal vesicles where they may regulate vesicular function. Although full-length presenilin 1 was present in crude homogenates, it was not detected in any of the vesicles studied, indicating that, unlike the presenilin fragments, full-length protein may not have a vesicular function.     

Designing structural-motifs for the preparation of acylated proinsulin and their regiospecific conversion into insulin modified at Lys29 (K29)

Eight proinsulin encoding genes were prepared and their translation products, when treated with a cocktail of trypsin and carboxypeptidase B, analyzed for the following features. One, their ability to undergo facile removal of the N-terminal linker, generating the phenylalanine residue destined to be the N-terminal of the B-chain of insulin, at a rate similar to that involved in the removal of the C-peptide. Two, processing of diarginyl insulin, produced in the latter process, by carboxypeptidase B then needed to be rapid to remove the two arginine residues, Three, both these operations were to be efficient whether the N-terminal methionine was acylated or not. Four, the proinsulin constructs needed to contain a minimum number of sites for acylation. The aforementioned features were monitored by mass spectrometry and the proinsulin derivative containing MRR at the N-terminal and K⁶⁴ mutated to Q⁶⁴, designated as MRR-(Q⁶⁴) human proinsulin [MRR-(Q⁶⁴) hpi] optimally fulfilled these requirements. The derivative was smoothly acylated with reagents of two chain lengths (acetyl and dodecanoyl) to give acetyl/dodecanoyl MRR-(Q⁶⁴) hpi. Acetyl MRR-(Q⁶⁴) hpi, using the cocktail of the two enzymes, was smoothly converted into, acetyl insulin. However, when dodecanoyl MRR-(Q⁶⁴) hpi was processed with the above cocktail, carboxypeptidase B (whether from animal pancreas or recombinant) showed an unexpected specificity of acting on the K²⁹-T³⁰ bond of the insulin derivatives when K²⁹ contained a large hydrophobic acyl group, generating dodecanoyl des-30 insulin.     

Lithium enhances secretion from large dense-core vesicles in nerve growth factor-differentiated PC12 cells

Considerable attention has been focused on the therapeutic role of lithium (Li) in bipolar disorders. Although no consensus has emerged, Li presumably influences the behavior of neurons that regulate mood and behavior. Using PC12 cells to study cellular and molecular actions of Li, we previously reported that Li modulates the expression of proteins associated with large dense-core vesicles (LDCVs; organelles typically containing monoamines, neuropeptides and other cargo proteins). The current investigation indicates that this enhanced expression of LDCV proteins correlates with an altered secretory phenotype in Li-treated cells. Immunoblotting detects significant increases in the cellular content and secretion of the LDCV cargo proteins chromogranin B and secretogranin II. Amperometry reveals an increase of spike number elicited by K+-depolarization of Li-treated cells but no change of spike amplitude or kinetics. Electron microscopy reveals no significant change in LDCV number per unit area in Li-treated cells. However, there is a significant increase (about 15%) in the diameter of LDCVs after Li. Thus, Li induces changes in the properties of LDCVs that culminate in augmented regulated secretion in nerve growth factor-differentiated PC12 cells. These results extend our understanding of Li-dependent changes of cellular function that may be germane to the therapeutic action of Li.     

Amphetamine reduces vesicular dopamine content in dexamethasone-differentiated PC12 cells only following l-DOPA exposure

Amphetamine (AMPH) increases brain dopamine (DA) levels via reversal of the membrane DA transporter. Additional mechanisms have been suggested, including inhibition of vesicular monoamine transporters and vesicular leakage of DA and Ca²⁺. According to the widely-accepted weak base theory, AMPH disrupts the proton gradient required for filling vesicles with DA. As a result, DA and Ca²⁺ will leak from vesicles, giving rise to exocytosis of less-filled vesicles. As several contradictions have been described, the aim of the present study was to re-examine this theory using amperometry and Fura-2 imaging to measure AMPH-induced changes in exocytosis and intracellular Ca²⁺ levels, respectively, in PC12 and chromaffin cells. Unexpectedly, 15 min exposure to AMPH (20–200 μM) does not affect the amount of DA released per vesicle, the frequency of exocytosis or intracellular Ca²⁺ levels in PC12 cells or chromaffin cells. Comparable results were found following prolonged exposure to AMPH (45 min) or at 37°C. When cells were pre-treated with the DA precursor l -DOPA, vesicle content increased to ∼150%. When these pre-treated cells are exposed to AMPH, vesicle content is strongly reduced. These results indicate that in dexamethasone-differentiated PC12 cells AMPH-induced vesicle leakage occurs only under specific conditions, therefore arguing for re-evaluation of the theory of AMPH-induced vesicular DA leakage.     

Specific binding of the C-peptide of proinsulin to cultured B-cells from a transplantable rat islet cell tumor

Specific binding of the C-peptide of proinsulin was evaluated using a transplantable NEDH rat islet cell tumour predominantly composed of insulin-secreting B-cells. Cultured tumour B-cells exhibited greater than 90% viability assessed by trypan blue exclusion, and retained the ability to form tumours with accompanying hypoglycaemia and hyperinsulinaemia after reimplantation. During binding experiments with synthetic rat C-peptide I and iodinated tyrosylated rat C-peptide I, turnout B-cells exhibited 54±6% specific binding. Displacement of tracer increased with increasing concentrations of unlabelled rat C-peptide I (0.25–1,000 ng/ml), and the specificity of binding was substantiated by reduced displacement with human C-peptide. Scatchard analysis of specific C-peptide binding revealed a curvilinear plot with upward concavity. The demonstration of specific C-peptide binding to insulin-secreting B-cells provides evidence for a physiological role of proinsulin C-peptide.     

Glycoprotein III (Clusterin, Sulfated Glycoprotein 2) in Endocrine, Nervous, and Other Tissues: Immunochemical Characterization, Subcellular Localization, and Regulation of Biosynthesis

Abstract: Specific antisera were raised against the A and B chains of glycoprotein III. lmmunoblotting revealed that in adrenal medulla both chains migrate very closely together in two-dimensional electrophoresis. Both chains with slightly differing molecular sizes are found in several endocrine tissues and in brain, kidney, liver, and serum. The mRNA has an analogous widespread distribution. In primary cultures of chromaffin cells the level of message becomes significantly increased by treatment with hista-mine or 12-O-tetradecanoylphorbol 13-acetate/forskoIin. However, the increase is small when compared with that of secretogranin II. The subcellular localization of glycoprotein III in endocrine organs and in the posterior pituitary was investigated by subcellular fractionation and immuno-electron microscopy. Glycoprotein III was found to be confined to the large densecore vesicles of these organs. For a discussion of the function of glycoprotein III, its localization in these organelles has to be taken into account.     

参与植物膜泡运输的蛋白质家族及功能研究进展

真核细胞的内吞和分泌途径中蛋白质和脂类的运输主要由膜泡运输介导。参与膜泡运输的蛋白质家族包括SNARE蛋白家族、RAB蛋白家族、被膜蛋白复合体、Sec1蛋白家族、Arf蛋白家族。这些蛋白质家族在进化中高度保守,并且在植物中已经鉴定了许多哺乳动物和酵母蛋白的同源物。近年来一些研究发现这些蛋白质不仅仅调节植物细胞的膜泡运输,还影响植物的许多生理活动和功能,例如向重性生长、胞质分裂、激素极性运输、气孔运动以及抗病性等。现主要阐述迄今在植物中研究这五类蛋白质家族功能的最新进展。