The Septate Junction Protein Tsp2A Restricts Intestinal Stem Cell Activity via Endocytic Regulation of aPKC and Hippo Signaling

Hippo signaling and the activity of its transcriptional coactivator, Yorkie (Yki), are conserved and crucial regulators of tissue homeostasis. In the Drosophila midgut, after tissue damage, Yki activity increases to stimulate stem cell proliferation, but how Yki activity is turned off once the tissue is repaired is unknown. From an RNAi screen, we identified the septate junction (SJ) protein tetraspanin 2A (Tsp2A) as a tumor suppressor. Tsp2A undergoes internalization to facilitate the endocytic degradation of atypical protein kinase C (aPKC), a negative regulator of Hippo signaling. In the Drosophila midgut epithelium, adherens junctions (AJs) and SJs are prominent in intestinal stem cells or enteroblasts (ISCs or EBs) and enterocytes (ECs), respectively. We show that when ISCs differentiate toward ECs, Tsp2A is produced, participates in SJ assembly, and turns off aPKC and Yki-JAK-Stat activity. Altogether, our study uncovers a mechanism allowing the midgut to restore Hippo signaling and restrict proliferation once tissue repair is accomplished. [Display omitted] •The assembly of septate junctions (SJs) occurs during ISC-EC differentiation•The SJ protein Tsp2A undergoes internalization and mediates aPKC degradation•Normal Tsp2A-SJ assembly ensures Hippo signaling to restrict ISC proliferation•Defective Tsp2A-SJ assembly causes aPKC accumulation and Yki hyperactivity SJ assembly is a hallmark of EC differentiation in the Drosophila midgut. Xu et al. identify SJ proteins as potent tumor suppressors and uncover Hippo signaling as the surveillance mechanism for SJ deficiency. Specifically, they demonstrate that SJ protein Tsp2A facilitates the endocytic degradation of the Hippo-pathway-antagonizing molecule aPKC.