Genetic and Molecular Characterization of ISWI Family Chromatin Remodelers in Wingless Signaling.

Abstract

The Wnt signaling pathway is highly conserved in the animal kingdom, and plays pivotal roles in cell fate specification throughout development and stem cell maintenance in adult tissues. Misregulation of the pathway has been causally linked to several cancers and osteoporosis. Research on how Wnt signaling regulates its target gene expression is important for us to understand its role in development and disease. In the absence of Wnt signaling, Wnt target genes are silenced by the DNA-binding protein TCF with the aid of several co-repressors, though the repression mechanism remains nebulous. The goal of my dissertation is to gain more knowledge on Wnt target gene repression by analyzing several ISWI family chromatin remodeling proteins including ISWI, ACF1 and Tou in Drosophila. ACF1 and Tou are closely related proteins, and they both associate with ISWI. By performing loss-of-function analysis, I found that ACF1 and Tou are not essential factors in Wg (Wingless, the Drosophila Wnt) signaling, yet they cooperatively repress Wnt target genes in cultured cells. Importantly, ISWI and ACF1 are required for the basal repression of several Wg target genes in both the developing wing and cultured cells. The repression by ISWI/ACF1 has substantial specificity for Wg targets and is different from its previously reported role in chromatin assembly and maintenance. I also observed that distinct from the binding profile of TCF, which is localized to the Wg regulated enhancers, ACF1 binds broadly across several Wg targets. The ACF1 binding is reduced upon Wg stimulation, possibly due to a widespread acetylation of chromatin. Synthesizing these observations, we propose a model where ISWI/ACF1 silences Wg target expression in unstimulated cells, and Wg signaling induces histone acetylation to displace/inactivate ISWI/ACF1 for gene activation. This work provides new insights on the transcriptional regulation of Wnt targets in context of chromatin.