Mechanistic regulation of Yes-associated protein (YAP) by mechanical forces and cellular density

Abstract

Yes-associated protein (YAP) and myocardin-related transcription factor (MRTF) play similar roles and exhibit significant crosstalk in directing transcriptional responses to chemical and physical extracellular cues. The mechanism underlying this crosstalk, however, remains unclear. Here, I show that MRTF family proteins bind YAP, and thereby recruit NcoA3 to the YAP-TEAD transcriptional complex to potentiate its transcriptional activity. I also demonstrate that this functional module is critical for cancer cell invasion in vitro and breast cancer metastasis to the lung $in vivo$. Lastly, I also detail the significance of MRTF-YAP binding in mechanotransduction, in which acute changes in the actin cytoskeleton induce nucleo-cytoplasmic shuttling of MRTF which underlies the LATS-independent regulation of YAP activity. Another actin-based, but conceptually distinct upstream stimulus that affects Hippo-YAP signaling is contact inhibition, which essentially prevents cells from hyper-proliferating upon reaching confluence. In this context, I also provide evidence that interaction between mono-ubiquitinated AMOTL2 and LATS2 facilitates recruitment of upstream Hippo pathway components such as SAV1. Via manual $in vivo$ ubiquitination screening, I identified WWP1 as the E3 ligase which mono-ubiquitinates AMOTL2. Functionally speaking, WWP1 knockdown hindered LATS activity by displacing its interaction with AMOTL2, consequently leading to YAP nuclear translocation and transcriptional activation. Furthermore, I found that WWP1 protein was stabilized under high cellular density, which occurred specifically at apical junctions where it interacted with the Crumbs polarity proteins. Altogether, I delineate two independent cellular mechanisms upstream and downstream of core Hippo signaling in which mechanical stress and cellular architecture modulate YAP activity.