The BEAF-32 insulator protein is required for Hippo pathway activity in the terminal differentiation of neuronal subtypes

The Hippo pathway is critical for not only normal growth and apoptosis but also cell fate specification during development. What controls Hippo pathway activity during cell fate specification is incompletely understood. In this research report, we identify the BEAF-32 insulator protein as a regulator of Hippo pathway activity in Drosophila photoreceptor differentiation. Though morphologically uniform, the fly eye is composed of two subtypes of R8 photoreceptor neurons defined by expression of light-detecting Rhodopsin proteins. In one R8 subtype, active Hippo signaling induces Rhodopsin6 (Rh6) and represses Rhodopsin5 (Rh5) whereas in the alternate subtype, inactive Hippo signaling induces Rh5 and represses Rh6. The activity state of the Hippo pathway in R8 is determined by the expression of warts, a core pathway kinase, which interacts with the growth regulator melted in a double negative feedback loop. We show that the BEAF-32 insulator is required for expression of warts and repression of melted. Furthermore, BEAF-32 plays a second role downstream of Warts to induce Rh6 and prevent Rh5 fate. BEAF-32 is dispensable for Warts feedback, indicating that BEAF-32 differentially regulates warts and Rhodopsins. Loss of BEAF-32 does not noticeably impair the functions of the Hippo pathway in eye growth regulation. Our study identifies a context-specific regulator of Hippo pathway activity in post-mitotic neuronal fate, and reveals a developmentally specific role for a broadly expressed insulator protein.