Elucidating distinctive roles of Hippo pathway members YAP and TAZ in human in vitro neurogenesis

The transcriptional co-activator Yes-associated protein-1 (YAP) and its paralog WW domain-containing transcription regulator 1 (WWTR1, TAZ) are known to regulate organ size, proliferation, and stem cell signaling in multiple tissue systems. However, their roles in nervous system development and neuronal differentiation have been understudied. Here, we set out to characterize YAP/TAZ expression changes and function in human neurogenesis in a cell lineage-specific and/or developmental stage-specific manner. To this end, YAP and TAZ knockouts were created in human embryonic stem cells using CRISPR, neural differentiation was induced via dual SMAD inhibition protocol, and phenotypic changes from pluripotency to neural phenotypes were detected by immunofluorescence and flow cytometry. Gene expression of YAP and TAZ and their isoforms were determined with qPCR at the pluripotent and neural stem cell stages, as well as in neurons. Interestingly, both overall YAP and TAZ appeared to be upregulated during differentiation from pluripotency to neural stem cells. However, during differentiation towards neurons, YAP was downregulated while TAZ remained upregulated compared to the pluripotent stage. There was no significant change in the relative expressions of the individual isoforms studied so far during differentiation. Thus, while YAP and TAZ have largely been considered functionally identical, our findings indicate that during neural differentiation, their roles vary. Further study of the functions of YAP and TAZ in neural development and regeneration may prove beneficial for elucidating and controlling neural regenerative potential in vitro and in vivo.