Characterization of Hippo pathway regulation and the physiological implications of its downstream effectors YAP and TAZ

The Hippo pathway and its downstream effectors, transcriptional coactivators YAP and TAZ, are important for regulating tissue homeostasis and their dysregulation has been implicated in many human diseases and cancer. However, it is not clear how the Hippo pathway becomes dysregulated because few mutations have been identified in Hippo pathway components. Therefore, recent work in the Hippo field has focused on identifying upstream regulators of the Hippo pathway. Nevertheless, it is not always clear which components are most physiologically relevant in regulating YAP/TAZ. To provide an overview of which components are most physiologically relevant in regulating YAP/TAZ, we used CRISPR/Cas9 to create knockout cell lines for many of these components and tested their responses to a variety of physiological signals to determine which components are most critical in regulating YAP/TAZ. By this approach, we demonstrate that NF2 and RHOA are important regulators of YAP/TAZ, and TAOK1/3 are direct kinases for LATS1/2.Additionally, YAP and TAZ are traditionally viewed as being largely redundant, although there are evolutionary, structural, and physiological differences that suggest there may be differences in how they are regulated and in their downstream functions. To delineate any differences between YAP and TAZ, we compared LATS1/2 KO cells, in which YAP/TAZ are constitutively-active, YAP KO, TAZ KO, and YAP/TAZ KO cells, in which YAP/TAZ are constitutively-inactive. We found that inactivation of YAP had a much greater negative effect on many measures of cell physiology, including cell spreading, cell volume, glucose uptake and metabolism, cell proliferation, and migration, while YAP activation in the LATS1/2 KO cells had the opposite effect. Differences between YAP and TAZ may be explained by differences in protein stability and expression, as YAP protein expression is much higher than that of TAZ. We also identified some differences in the transcriptional profiles induced by YAP and TAZ, suggesting that, although they are largely similar, there may be important distinctions between YAP and TAZ