The Role and Mechanisms of Dlg5 in the Regulation of the Hippo Signaling Pathway
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The Hippo signal transduction pathway plays a pivotal role in regulation of normal development, adult organ homeostasis, and cancer. At the core of the Hippo pathway, MST1/2 kinases phosphorylate and activate LATS1/2 kinases, which in turn phosphorylate and inactivate transcriptional co-activators YAP1 and TAZ. Phosphorylated YAP1 and TAZ are inhibited from entering the nucleus and are instead cytoplasmically retained or degraded. Cells monitor their microenvironment, by sensing local cell density and cellular polarity, and use Hippo signaling to regulate the maintenance of normal cell numbers necessary for tissue function. The mechanisms responsible for the connection between the cellular microenvironment and Hippo pathway are poorly understood. In this study, in collaboration with Dr. Andrew Emili's laboratory in the University of Toronto, I reveal a novel mechanistic connection between the Hippo signaling pathway and the apical-basal cell polarity protein DLG5. I found that DLG5 binds to MST1/2 kinases of the core Hippo pathway. Inactivation of Dlg5 in mice in vivo and in primary cells ex vivo results in increased Hippo pathway activity and decreased levels and activity of YAP1 and TAZ. Overexpression of DLG5 inhibits Hippo signaling and promotes the expression of Hippo pathway target genes. In genetic epistasis experiments in mice, Dlg5 shows a strong genetic interaction with both Yap1 and Taz (Wwtr1). Mechanistically, I found that DLG5 negatively regulates the Hippo pathway by inhibiting the binding between MST1/2 and LATS1/2. This study increases the understanding of the connection between apical-basal polarity and the Hippo pathway and identifies apical-basal polarity family protein DLG5 as a novel interactor and regulator of the Hippo signal transduction pathway.