Joint control of epidermal cell fate by Yorkie and Bonus.

Dysregulation of the Hippo pathway results in organ overgrowth and contributes to carcinogenesis in humans. Although the Hippo pathway has been intensively studied for its regulation of cell proliferation and organ growth, its role beyond growth control remains elusive. To explore possible novel functions of the Hippo pathway, we carried out an affinity purification-mass spectrometry (AP-MS) analysis in Drosophila embryos and cultured cells, using transcriptional coactivator Yorkie (Yki, YAP1 homolog) as bait. Prominent groups of identified interactors included all of the core pathway components and known accessory regulators, as well as subunits of RNA polymerase II and components of the Trithorax-related (Trr) complex. Among putative novel interactors we found Bonus (Bon), the Drosophila homolog of the mammalian transcriptional intermediary factor-1/tripartite motif containing (TIF1/TRIM24/TRIM33) family of proteins. Protein interaction between Bon and Yki requires the WW domains in Yki and PPxY motifs in Bon. Interestingly, Bon overexpression leads to formation of epidermal extensions (trichomes) on the surface of adult eyes. This phenotype requires Yki and Scalloped (Sd, TEAD homolog), as well as Bon-Yki interaction. To obtain a mechanistic understanding of this apparent switch towards an epidermal cell fate, we searched for novel genes that may be jointly regulated by Bon and Yki, and identified several candidates based on published ChIP-seq and RNA-seq analyses. These putative target genes are not canonical Hippo pathway effectors, but include factors involved in early eye specification as well as genes controlled by steroid hormone signaling. Loss or gain of function of a subset of these genes modified the trichome phenotype, suggesting that they are under the control of Bon and Yki and are involved in establishing a choice between a “generic” epidermal cell fate and eye field specification. In summary, our study reveals a novel function of the Hippo pathway in the regulation of epidermal cell differentiation. This regulation occurs through the interaction between Yki and Bon as well as their joint transcriptional control of nonclassical target genes. This work broadens our understanding of the Hippo pathway beyond its role in growth control. Citation Format: Heya Zhao, Can Zhang, Kenneth Moberg, Alexey Veraksa. Joint control of epidermal cell fate by Yorkie and Bonus [abstract]. In: Proceedings of the AACR Special Conference on the Hippo Pathway: Signaling, Cancer, and Beyond; 2019 May 8-11; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(8_Suppl):Abstract nr B30.