Department of Biology, York University, Toronto, ON, M3J 1P3, Canada.
Muscle Health Research Centre (MHRC), York University, Toronto, ON, M3J 1P3, Canada.
J Cell Sci. 2022 Jan 1;135(1). doi: 10.1242/jcs.259097. Epub 2022 Jan 12.
Hippo signaling in Drosophila and mammals is prominent in regulating cell proliferation, death and differentiation. Hippo signaling effectors (YAP and TAZ; also known as YAP1 and WWTR1, respectively) exhibit crosstalk with transforming growth factor-β (TGF-β)-Smad and Wnt/β-catenin pathways. Previously, we implicated Smad7 and β-catenin in mammalian myogenesis. Therefore, we assessed a potential role of TAZ on the Smad7-β-catenin complex in muscle cells. Here, we document functional interactions between Smad7, TAZ and β-catenin in mouse myogenic cells. Ectopic TAZ expression resulted in repression of the muscle-specific creatine kinase muscle (Ckm) gene promoter and its corresponding protein level. Depletion of endogenous TAZ enhanced Ckm promoter activation. Ectopic TAZ, while potently active on a TEAD reporter (HIP-HOP), repressed myogenin (Myog) and Myod1 enhancer regions and myogenin protein level. Additionally, a Wnt/β-catenin readout (TOP flash) demonstrated TAZ-mediated inhibition of β-catenin activity. In myoblasts, TAZ was predominantly localized in nuclear speckles, while in differentiation conditions TAZ was hyperphosphorylated at Ser89, leading to enhanced cytoplasmic sequestration. Finally, live-cell imaging indicated that TAZ exhibits properties of liquid-liquid phase separation (LLPS). These observations indicate that TAZ, as an effector of Hippo signaling, suppresses the myogenic differentiation machinery.
果蝇和哺乳动物中的 Hippo 信号通路在调节细胞增殖、死亡和分化方面发挥着重要作用。Hippo 信号通路效应物（YAP 和 TAZ；也分别称为 YAP1 和 WWTR1）与转化生长因子-β（TGF-β）-Smad 和 Wnt/β-连环蛋白通路相互作用。先前，我们发现在哺乳动物的肌发生过程中 Smad7 和 β-连环蛋白起作用。因此，我们评估了 TAZ 在肌细胞中对 Smad7-β-连环蛋白复合物的潜在作用。在这里，我们记录了 Smad7、TAZ 和 β-连环蛋白在小鼠肌原细胞中的功能相互作用。异位表达 TAZ 导致肌肉特异性肌酸激酶肌肉（Ckm）基因启动子及其相应蛋白水平受到抑制。内源性 TAZ 的耗竭增强了 Ckm 启动子的激活。TAZ 虽然对 TEAD 报告基因（HIP-HOP）具有很强的活性，但抑制了肌球蛋白（Myog）和 Myod1 增强子区域以及肌球蛋白蛋白水平。此外，Wnt/β-连环蛋白读数（TOP flash）表明 TAZ 介导的β-连环蛋白活性抑制。在成肌细胞中，TAZ 主要定位于核斑点中，而在分化条件下，TAZ 在 Ser89 处被高度磷酸化，导致细胞质隔离增强。最后，活细胞成像表明 TAZ 表现出液-液相分离（LLPS）的特性。这些观察结果表明，作为 Hippo 信号通路的效应物，TAZ 抑制了成肌分化机制。
