Sanpodo, a novel four-pass transmembrane protein, regulates asymmetric precursor divisions in the Drosophila CNS

Neural diversity is the developmental foundation of a functional nervous system. A conserved, general strategy for creating cellular diversity is the asymmetric division of precursor cells to generate daughter cells with distinct fates. In the Drosophila embryonic CNS, a series of reiterative asymmetric neural precursor divisions create the diverse population of post-mitotic neurons that together form the basis of the elaborate network comprising the mature CNS. Initially, each neural stem cell divides asymmetrically to regenerate itself and produce a secondary precursor cell known as a ganglion mother cell (GMC). Each GMC then divides asymmetrically to yield two daughter neurons with distinct fates (A,B). Asymmetric GMC divisions are dependent upon the convergence of intrinsic and extrinsic cues: the presence of active Notch signaling in one sibling (A) and the numb-dependent inhibition of Notch signaling in the other (B) allows sibling neurons to acquire distinct fates.; sanpodo was identified as a key regulator of asymmetric GMC divisions through genetic screens, and was initially characterized as the actin-associated protein Tropomodulin. However, here I present evidence that sanpodo is not tropomodulin, and demonstrate that sanpodo encodes a novel four-pass transmembrane protein that functions upstream of Notch and downstream of Delta to promote asymmetric divisions. Antibodies specific to Sanpodo demonstrate that Sanpodo is expressed in asymmetrically dividing cells and their progeny; within these cells, Sanpodo localizes around the entire periphery of the cell membrane, in small aggregates on the cell membrane, and in cytoplasmic vesicles. Sanpodo localizes with Notch and immediately adjacent to Delta, and I observe a general inverse correlation between high level Numb expression and Sanpodo membrane localization. Analysis of the asymmetric division of a well-characterized CNS precursor reveals that Sanpodo internalization occurs preferentially in the B cell through a numb-dependent mechanism. Finally, Sanpodo binds Numb and Notch, but not Delta. Together these findings support a model in which Sanpodo acts with Notch at the membrane of the A cell to promote productive Notch signaling and the A fate. In the B cell, Numb removes Sanpodo from the membrane thus blocking active Notch signaling and enabling adoption of the B fate.