<i>Drosophila</i> Abi maintains blood cell homeostasis by promoting clathrin-mediated endocytosis of Notch

Abl-interactor (Abi) proteins induce actin polymerization by activating Wiskott-Aldrich syndrome protein (WASp) or SCAR/WASP-family verprolin-homologous protein. Loss of mammalian Abi1 causes myeloproliferative neoplasm; however, little is known about how the Abi family of actin-regulatory proteins regulates blood cell homeostasis. Here, we demonstrate that Drosophila Abi promotes plasmatocyte-to-crystal cell transdifferentiation but represses plasmatocyte-to-lamellocyte transdifferentiation through Notch signaling. Consistent with a previously demonstrated role of clathrin-mediated endocytosis (CME) in Notch signaling activation, we find that Abi promotes Notch-CME by recruiting WASp and the Notch receptor to nascent sites of CME. Finally, we demonstrate that CME and crystal cell formation are inhibited by Abelson (Abl)-mediated phosphorylation of Abi but require PTP61F, a phosphatase that reverses this phosphorylation. Our findings identify Abi as a critical integrator of actin remodeling and Notch-CME and reveal opposing roles of Abl and PTP61F in regulating Abi activity to maintain blood cell homeostasis.