TAZ exhibits phase separation properties and interacts with Smad7 and beta-catenin to repress skeletal myogenesis

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-beta (TGF-beta)-Smad and Wnt/beta-catenin pathways. Previously, we implicated Smad7 and beta-catenin in mammalian myogenesis. Therefore, we assessed a potential role of TAZ on the Smad7-beta-catenin complex in muscle cells. Here, we document functional interactions between Smad7, TAZ and beta-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/beta-catenin readout (TOP flash) demonstrated TAZ-mediated inhibition of beta-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.