Verbert Leen, Lee Bora, Kocks Sarah L, Assefa Zerihun, Parys Jan B, Missiaen Ludwig, Callewaert Geert, Fissore Rafael A, De Smedt Humbert, Bultynck Geert
Laboratory of Molecular and Cellular Signalling, Division of Physiology, Department of Molecular Cell Biology, K.U. Leuven, Campus Gasthuisberg, O&N1 bus 802, B-3000 Leuven, Belgium.
Biol Cell. 2008 Jan;100(1):39-49. doi: 10.1042/BC20070086.
The IP(3)R (inositol 1,4,5-trisphosphate receptor) is a tetrameric channel that accounts for a large part of the intracellular Ca(2+) release in virtually all cell types. We have previously demonstrated that caspase-3-mediated cleavage of IP(3)R1 during cell death generates a C-terminal fragment of 95 kDa comprising the complete channel domain. Expression of this truncated IP(3)R increases the cellular sensitivity to apoptotic stimuli, and it was postulated to be a constitutively active channel.
In the present study, we demonstrate that expression of the caspase-3-cleaved C-terminus of IP(3)R1 increased the rate of thapsigargin-mediated Ca(2+) leak and decreased the rate of Ca(2+) uptake into the ER (endoplasmic reticulum), although it was not sufficient by itself to deplete intracellular Ca(2+) stores. We detected the truncated IP(3)R1 in different cell types after a challenge with apoptotic stimuli, as well as in aged mouse oocytes. Injection of mRNA corresponding to the truncated IP(3)R1 blocked sperm factor-induced Ca(2+) oscillations and induced an apoptotic phenotype.
In the present study, we show that caspase-3-mediated truncation of IP(3)R1 enhanced the Ca(2+) leak from the ER. We suggest a model in which, in normal conditions, the increased Ca(2+) leak is largely compensated by enhanced Ca(2+)-uptake activity, whereas in situations where the cellular metabolism is compromised, as occurring in aging oocytes, the Ca(2+) leak acts as a feed-forward mechanism to divert the cell into apoptosis.
IP(3)R（肌醇1,4,5-三磷酸受体）是一种四聚体通道，几乎在所有细胞类型的细胞内钙释放中都占很大比例。我们之前已经证明，细胞死亡过程中caspase-3介导的IP(3)R1裂解会产生一个95 kDa的C末端片段，该片段包含完整的通道结构域。这种截短的IP(3)R的表达增加了细胞对凋亡刺激的敏感性，据推测它是一种组成型活性通道。
在本研究中，我们证明IP(3)R1的caspase-3裂解C末端的表达增加了毒胡萝卜素介导的钙泄漏速率，并降低了钙摄取到内质网（ER）中的速率，尽管它本身不足以耗尽细胞内钙库。在用凋亡刺激物刺激后，我们在不同细胞类型以及老龄小鼠卵母细胞中检测到了截短的IP(3)R1。注射与截短的IP(3)R1对应的mRNA可阻断精子因子诱导的钙振荡并诱导凋亡表型。
在本研究中，我们表明caspase-3介导的IP(3)R1截短增强了内质网的钙泄漏。我们提出了一个模型，即在正常情况下，增加的钙泄漏在很大程度上由增强的钙摄取活性所补偿，而在细胞代谢受损的情况下，如老龄卵母细胞中发生的情况，钙泄漏充当一种前馈机制，将细胞导向凋亡。
