Lipid transport by TMEM24 at ER-plasma membrane contacts regulates pulsatile insulin secretion.
Lipid transport by TMEM24 at ER-plasma membrane contacts regulates pulsatile insulin secretion.
复制标题
DOI：
10.1126/science.aah6171
复制
发表时间：
2017-02-17
期刊：
影响因子：
--
通讯作者：
Reinisch KM
中科院分区：
文献类型：
--
作者：
Lees JA;Messa M;Sun EW;Wheeler H;Torta F;Wenk MR;De Camilli P;Reinisch KM
胰岛素由胰腺β细胞响应于葡萄糖刺激而分泌。其释放受钙和磷酸肌醇信号通路的相互作用控制。快速释放阶段，其中已经在质膜（PM）处对接和引发的含有胰岛素的颗粒经历胞吐作用，随后是缓慢释放。在第二阶段，颗粒被对接和引发，然后以一系列爆发的方式释放，每个爆发都由胞质Ca 2+的尖峰触发。为了更好地了解胰岛素分泌的分子基础，我们表征了TMEM 24，一种富含神经内分泌细胞的蛋白质，以前认为是正常分泌反应所必需的。我们发现TMEM24是一种内质网（ER）蛋白，集中在ER-PM接触位点，在那里它连接两个双层。TMEM24与PM的"反式"结合受到响应于胞质Ca 2+升高的磷酸化的负调节，使得TMEM24随着Ca 2+浓度尖峰而与PM瞬时解离，然后在去磷酸化后与该膜重新缔合。此外，TMEM 24含有突触结合蛋白样的线粒体和脂质结合蛋白（SMP）家族的脂质转运模块，我们在结构上表征了该模块，并显示其优先结合磷脂酰肌醇（PI）的甘油脂。因此，TMEM24有助于将在ER中合成的PI递送至PM，在PM中其转化为磷脂酰肌醇-4，5-二磷酸[PIP2]以补充在葡萄糖刺激的信号传导期间水解的这种脂质的池。支持TMEM 24在Ca2+和磷酸肌醇信号传导的协调中的关键作用，TMEM 24的脂质转运功能对于维持细胞内Ca2+振荡是必不可少的，所述细胞内Ca2+振荡触发胰岛素颗粒释放的爆发并因此触发胰岛素分泌。PIP2是Ca2+依赖性胞吐作用所必需的。它还控制PM离子通道的活性，PM离子通道调节细胞溶质Ca2+水平，并且是IP3的前体，IP3也有助于通过触发Ca 2+从ER释放来调节细胞溶质Ca 2+。因此，在胰岛素分泌细胞中，TMEM 24参与协调Ca2+和磷酸肌醇信号通路，以引起脉冲式胰岛素分泌（见图）。我们的研究结果牵连ER-PM接触网站和ER居民脂质转移蛋白在PM磷酸肌醇池的直接调节，提供新的见解细胞磷酸肌醇动力学的机制。更具体地说，他们阐述了胰岛素分泌的机制，这是受损的II型糖尿病患者，并可能最终有治疗的分歧。胰岛素分泌细胞的葡萄糖刺激触发Ca2+内流、磷脂酶C依赖性PIP2裂解和颗粒胞吐。Ca2+刺激的磷酸化导致TMEM 24从PM解离并中断SMP介导的PI转移，从而允许PIP2再合成。降低的PIP2水平减弱兴奋性反应。去磷酸化允许TMEM 24返回PM以补充PIP2，从而允许Ca2+升高和分泌的新循环。
Insulin is secreted by pancreatic β cells in response to glucose stimulation. Its release is controlled by the interplay of calcium and phosphoinositide signaling pathways. A rapid release phase, in which insulin containing granules that are already docked and primed at the plasma membrane (PM) undergo exocytosis, is followed by slow release. In this second phase, granules are docked and primed and then released in a series of bursts, each triggered by a spike in cytosolic Ca2+. To better understand the molecular basis underlying insulin secretion, we characterized TMEM24, a protein enriched in neuroendocrine cells previously suggested to be required for a normal secretory response. We found that TMEM24 is an endoplasmic reticulum (ER) protein that concentrates at ER-PM contact sites, where it tethers the two bilayers. TMEM24 binding “in trans” to the PM is negatively regulated by phosphorylation in response to elevation of cytosolic Ca2+, so that TMEM24 transiently dissociates from the PM as Ca2+ concentration spikes and then reassociates with this membrane upon dephosphorylation. Additionally, TMEM24 contains a lipid transport module of the synaptotagmin-like, mitochondrial and lipid-binding protein (SMP) family, which we structurally characterized and showed to bind glycerolipids with a preference for phosphatidylinositol (PI). Thus, TMEM24 helps deliver PI, which is synthesized in the ER, to the PM, where it is converted to phosphatidylinositol-4,5-bisphosphate [PIP2] to replenish pools of this lipid hydrolyzed during glucose-stimulated signaling. Supporting a key role of TMEM24 in the coordination of Ca2+ and phosphoinositide signaling, the lipid transport function of TMEM24 is essential for sustaining the intracellular Ca2+ oscillations that trigger bursts of insulin granule release and hence insulin secretion. PIP2 is required for Ca2+-dependent exocytosis. It also controls the activity of PM ion channels that regulate cytosolic Ca2+ levels and is the precursor of IP3, which also helps to modulate cytosolic Ca2+ by triggering Ca2+ release from the ER. Thus, in insulin-secreting cells, TMEM24 participates in coordinating Ca2+ and phosphoinositide signaling pathways to cause pulsatile insulin secretion (see the figure). Our findings implicate ER-PM contact sites and an ER resident lipid-transfer protein in the direct regulation of PM phosphoinositide pools, offering fresh insights into the mechanisms of cellular phosphoinositide dynamics. More specifically, they elaborate the mechanisms underlying insulin secretion, which is impaired in patients with type II diabetes, and may ultimately have therapeutic ramifications. Glucose stimulation of insulin-secreting cells triggers Ca2+ influx, phospholipase C–dependent PIP2 cleavage, and granule exocytosis. Ca2+-stimulated phosphorylation causes TMEM24 dissociation from the PM and interruption of SMP-mediated PI transfer that allows PIP2 resynthesis. Lowered PIP2 levels attenuate the excitatory response. Dephosphorylation allows TMEM24 to return to the PM to replenish PIP2, permitting a new cycle of Ca2+ elevation and secretion.