Calcium-induced movement of troponin-I relative to actin in skeletal muscle thin filaments

The role of troponin-I (the inhibitory subunit of troponin) in the regulation by Ca2+ of skeletal muscle contraction was investigated with resonance energy transfer and photo cross-linking techniques. The effect of Ca2+ on the proximity of troponin-I to actin in reconstituted rabbit skeletal thin filaments was determined. The distance between the cysteine residue at position 133 (Cys133) of troponin-I and Cys374 of actin increases by approximately 15 angstroms on binding of Ca2+ to troponin-C. Also, troponin-I labeled at Cys133 with benzophenone-4-maleimide could be photo cross-linked to actin in the absence of Ca2+, but not in its presence. These results suggest that troponin-I is attached to actin in the Ca2(+)-free or relaxed state of muscle, and that it detaches from actin on Ca2+ activation of contraction. Thus, troponin-I may function as a Ca2(+)-dependent molecular switch in regulation of skeletal muscle contraction.     

Subcellular calcium transients visualized by confocal microscopy in a voltage-clamped vertebrate neuron

Confocal laser-scanned microscopy and long-wavelength calcium (Ca2+) indicators were combined to monitor both sustained and rapidly dissipating Ca2+ gradients in voltage-clamped sympathetic neurons isolated from the bullfrog. After a brief activation of voltage-dependent Ca2+ channels, Ca2+ spreads inwardly, and reaches the center of these spherical cells in about 300 milliseconds. Although the Ca2+ redistribution in the bulk of the cytosol could be accounted for with a radial diffusion model, local nonlinearities, suggesting either nonuniform Ca2+ entry or spatial buffering, could be seen. After electrical stimulation, Ca2+ signals in the nucleus were consistently larger and decayed more slowly than those in the cytosol. A similar behavior was observed when release of intracellular Ca2+ was induced by caffeine, suggesting that in both cases large responses originate from Ca2+ release sites near or within the nucleus. These results are consistent with an amplification mechanism involving Ca2(+)-induced Ca2+ release, which could be relevant to activity-dependent, Ca2(+)-regulated nuclear events.     

四种金属离子对地图幼鱼淀粉酶活性的影响

以酶学分析的方法研究了4种金属离子Na＋、K＋、Ca2＋、Mg2＋分别在单因子和双因子条件下对地图幼鱼淀粉酶活性的影响。结果表明：在设定的浓度梯度内,单因子条件下,除Ca2＋对淀粉酶活性起强烈的抑制作用外,其余3种离子对淀粉酶活性均起促进作用,其中Mg2＋的促进作用最强。双因子条件下,Na＋和K＋互作对淀粉酶的激活作用高于2者单独作用,Na＋和Mg2＋在一定浓度下互作对淀粉酶的激活作用高于2者单独作用,而K＋和Mg2＋互作则会抑制Mg2＋的激活作用。     

Postsynaptic Induction of BDNF-Mediated Long-Term Potentiation

Brain-derived neurotrophic factor (BDNF) and other neurotrophins are critically involved in long-term potentiation (LTP). Previous reports point to a presynaptic site of neurotrophin action. By imaging dentate granule cells in mouse hippocampal slices, we identified BDNF-evoked Ca2+ transients in dendrites and spines, but not at presynaptic sites. Pairing a weak burst of synaptic stimulation with a brief dendritic BDNF application caused an immediate and robust induction of LTP. LTP induction required activation of postsynaptic Ca2+ channels and N-methyl-d-aspartate receptors and was prevented by the blockage of postsynaptic Ca2+ transients. Thus, our results suggest that BDNF-mediated LTP is induced postsynaptically. Our finding that dendritic spines are the exclusive synaptic sites for rapid BDNF-evoked Ca2+ signaling supports this conclusion.     

水涝胁迫下海滨锦葵幼苗根尖细胞内Ca~(2+)分布与变化

采用醋酸双氧铀染色与透射电镜技术,以海滨锦葵为试验材料,连续观测水涝胁迫下及涝后恢复期间海滨锦葵根尖细胞内Ca~(2+)的分布与变化特性。结果显示:随着水涝时间延长,海滨锦葵根尖细胞间隙与细胞核、液泡中钙离子沉积密度逐步降低,质体外膜上存在Ca~(2+)分布,但低于对照组,而Ca~(2+)向细胞质移动,在局部区域聚集,导致细胞质钙离子增加。水涝去除20 d后,细胞壁中出现钙离子沉积,细胞间隙、质体外膜上与液泡中所分布钙离子增加,细胞质所聚集的Ca~(2+)逐步分散,基本不存在Ca~(2+)沉积。研究认为,水涝胁迫下,海滨锦葵根尖细胞内Ca~(2+)浓度迅速上升;涝后恢复期间,Ca~(2+)浓度逐渐下降,起着外界信号传递的作用。     

A magnesium current in Paramecium

Recent reappraisals of the role of ionized magnesium in cell function suggest that many cells maintain intracellular free Mg2+ at low concentrations (0.1 to 0.7 mM) and that external agents can influence cell function via changes in intracellular Mg2+ concentration. Depolarization and hyperpolarization of voltage-clamped Paramecium elicited a Mg2(+)-specific current, IMg. Both Co2+ and Mn2+ were able to substitute for Mg2+ as charge carriers, but the resultant currents were reduced compared with Mg2+ currents. Intracellular free Mg2+ concentrations were estimated from the reversal potential of IMg to be about 0.39 mM. The IMg was inhibited when external Ca2+ was removed or a Ca2+ chelator was injected, suggesting that its activation was Ca2(+)-dependent.     

Immunocyte Ca2+ influx system mediated by LTRPC2

We characterized an activation mechanism of the human LTRPC2 protein, a member of the transient receptor potential family of ion channels, and demonstrated that LTRPC2 mediates Ca2+ influx into immunocytes. Intracellular pyrimidine nucleotides, adenosine 5'-diphosphoribose (ADPR), and nicotinamide adenine dinucleotide (NAD), directly activated LTRPC2, which functioned as a Ca2+-permeable nonselective cation channel and enabled Ca2+ influx into cells. This activation was suppressed by intracellular adenosine triphosphate. These results reveal that ADPR and NAD act as intracellular messengers and may have an important role in Ca2+ influx by activating LTRPC2 in immunocytes.     

BKCa-Cav channel complexes mediate rapid and localized Ca2+-activated K+ signaling

Large-conductance calcium- and voltage-activated potassium channels (BKCa) are dually activated by membrane depolarization and elevation of cytosolic calcium ions (Ca2+). Under normal cellular conditions, BKCa channel activation requires Ca2+ concentrations that typically occur in close proximity to Ca2+ sources. We show that BKCa channels affinity-purified from rat brain are assembled into macromolecular complexes with the voltage-gated calcium channels Cav1.2 (L-type), Cav2.1 (P/Q-type), and Cav2.2 (N-type). Heterologously expressed BKCa-Cav complexes reconstitute a functional "Ca2+ nanodomain" where Ca2+ influx through the Cav channel activates BKCa in the physiological voltage range with submillisecond kinetics. Complex formation with distinct Cav channels enables BKCa-mediated membrane hyperpolarization that controls neuronal firing pattern and release of hormones and transmitters in the central nervous system.     

钙离子在介导SA 诱导白桦悬浮细胞三萜合成途径中的作用

为探讨Ca 2 + 作为信号分子在水杨酸(SA)诱导白桦悬浮细胞三萜合成中的作用,本文用SA、Ca 2 + 及Ca 2 + 阻断 剂对白桦悬浮细胞进行处理,采用紫外分光光度法和高效液相色谱法测定细胞中总三萜及其组分的积累量;同时 利用荧光定量PCR 方法检测了白桦细胞中三萜合成途径关键酶基因(FPS、SS、SE、BPW、BPY、CAS)和钙调蛋白基 因表达变化。结果显示:水杨酸可诱导白桦悬浮细胞三萜物质(总三萜、齐墩果酸、白桦脂醇、白桦脂酸)的合成及 相关基因的上调表达,且Ca 2 + 的加入使得这种诱导效应更为显著,暗示白桦细胞中钙信号分子介导了水杨酸诱导 三萜物质的合成。而对SA 与各种Ca 2 + 阻断剂互作处理结果显示,Ca 2 + 抑制剂的加入抑制了SA 对细胞中三萜合 成的诱导效果,表明Ca 2 + 作为信号分子介导了SA 诱导白桦悬浮细胞三萜物质的合成。      

金属离子对4PCA饲料酶中木聚糖酶活性的影响

[目的]为酶制剂的稳定性研究及其在饲料工业中的应用提供依据。[方法]选取K+、Na+、Ca2+、Mg2+、Zn2+、Mn2+、Cu2+和Fe2+8种金属离子,浓度设计为1×10-4、1×10-3、1×10-2mol/L,研究金属离子对4PCA饲料复合酶中木聚糖酶活性的影响。[结果]浓度为1×10-4mol/L时,8种金属离子对木聚糖酶活性无明显影响。浓度为1×10-3mol/L时,Cu2+使木聚糖酶活增至125.8%,Mn2+使木聚糖酶活降至54.5%。浓度为1×10-2mol/L时,Zn2+使木聚糖酶活增至121.5%,Mn2+、Cu2+、Fe2+分别使木聚糖酶活降至40.6%、67.7%、88.9%。[结论]K+、Na+、Ca2+、Mg2+对木聚糖酶活性无特别影响,Zn2+有激活作用,Mn2+、Fe2+有抑制作用,Cu2+随着浓度的增加有先激活后抑制作用,3种离子的抑制作用由大到小依次为:Mn2+>Cu2+>Fe2+。     

TCR-induced transmembrane signaling by peptide/MHC class II via associated Ig-alpha/beta dimers

Previous findings suggest that during cognate T cell-B cell interactions, major histocompatability complex (MHC) class II molecules transduce signals, leading to Src-family kinase activation, Ca2+ mobilization, and proliferation. Here, we show that antigen stimulation of resting B cells induces MHC class II molecules to associate with Immunoglobulin (Ig)-alpha/Ig-beta (CD79a/CD79b) heterodimers, which function as signal transducers upon MHC class II aggregation by the T cell receptor (TCR). The B cell receptor (BCR) and MHC class II/Ig-alpha/Ig-beta are distinct complexes, yet class II-associated Ig-alpha/beta appears to be derived from BCR. Hence, Ig-alpha/beta are used in a sequential fashion for transduction of antigen and cognate T cell help signals.     

gCap39, a calcium ion- and polyphosphoinositide-regulated actin capping protein

The polymerization of actin filaments is involved in growth, movement, and cell division. It has been shown that actin polymerization is controlled by gelsolin, whose interactions with actin are activated by calcium ion (Ca2+) and inhibited by membrane polyphosphoinositides (PPI). A smaller Ca2(+)- and PPI-regulated protein, gCap39, which has 49% sequence identity with gelsolin, has been identified by cDNA cloning and protein purification. Like gelsolin, gCap39 binds to the fast-growing (+) end of actin filaments. However, gCap39 does not sever actin filaments and can respond to Ca2+ and PPI transients independently, under conditions in which gelsolin is ineffective. The coexistence of gCap39 with gelsolin should allow precise regulation of actin assembly at the leading edge of the cell.