Regulation and feedback in Fat/Dachsous signaling

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The giant Drosophila protocadherins Fat and Dachsous (Ds) form a heterophilic, bidirectional signaling pair that regulates proliferation via the growth-inhibiting Hippo pathway, and planar cell polarity (PCP) both through and independently of the “core” PCP pathway. These functions are shared by their mammalian homologs, and human mutations in Fat and Ds homologs cause the neurological and multisystem defects of Hennekam and Van Maldergem syndromes. Despite its importance, only a little is known about how binding between Fat and Ds change cell behavior, and thus how it regulates development and pathology. Fat, Ds and the effectors of the Hippo and PCP pathways are concentrated in the subapical domain of epithelial cells, and the intracellular domain (ICD) of Fat has strong effects on the subapical levels of two critical proteins. The first is the scaffolding myosin Dachs, which binds and inhibits Warts (Lats1/2), the final effector kinase in the Hippo pathway, and which regulates Sple in the core PCP pathway. The second is the FERM scaffolding protein Expanded, which stimulates Warts activity. Using a combination of protein-binding screens, biochemistry and molecular genetics, we established a link from the Fat ICD to Dachs and Expanded levels and localization via the DHHC palmitoyltransferase Approximated and one of its targets, the SH3 domain protein Dlish. However, Approximated must have additional targets in the pathway, and the details of Approximated and Dlish regulation are poorly understood. In this proposal we outline plans to identify characterize new Approximated targets, including Expanded itself, and further plans to analyze the regulation of Approximated activity and target recognition, first through modification of Approximated and its targets, and second through the previously uncharacterized GOLGA7-like adapter protein CG5447. Fat and Ds are also remarkable in their ability to polarize cells along the epithelial plane via their own cell-by- cell polarization to opposite cell faces. We have initiated studies on the intracellular control and amplification of Fat/Ds polarization. We explore previously unsuspected roles for intracellular pathway components in the regulation of Fat and Ds levels and polarization, including the casein kinse 1 Dco, the ubiquitin ligase Slimb, the myosin Dachs, and the intracellular domain of Fat itself.