Follicle cell expression of the Drosophila pipe gene defines dorsal -ventral polarity in the embryo

Abstract

Determination of dorsal-ventral (DV) polarity in the Drosophila embryo involves two sequentially acting signal transduction pathways. The DV polarity of the egg chamber is specified by a localized signal from the oocyte nucleus that defines polarity in the follicle cell layer. Subsequently, the embryonic DV axis is determined by the localized activation of a proteolytic cascade in the ventral perivitelline space, a process that requires prior establishment of DV polarity in the follicle. Three of the 12 maternal effect loci required for the establishment of DV polarity in the embryo namely pipe, wind and nudel are required in the soma suggesting that one or more of these genes could link follicle cell and embryonic DV polarity. This study was initiated to clone and characterize the pipe gene to gain more knowledge about its function in the establishment of embryonic DV polarity. The following have been demonstrated during the course of this study: (1) pipe is expressed in a ventrally restricted pattern in the follicle cells. (2) Mutations that alter egg chamber polarity alter the distribution of pipe in the follicle. (3) Directed expression of pipe in follicle cells of females otherwise mutant for pipe can restore ventral and lateral pattern elements to their progeny embryos. (4) pipe encodes a protein that is homologous to the carbohydrate-modifying enzyme Heparan sulfate 2-O-sulfotransferase (HS2ST) from Chinese Hamster Ovary cells. The pipe gene and protein were further characterized by sequencing the 11 EMS-generated alleles of pipe to identify functionally important regions of the protein. As a part of functional studies stable cell lines expressing FLAG-tagged Pipe in CHO cells mutant for HS2ST were generated to test the ability of pipe to functionally substitute for CHO-HS2ST. In these cells Pipe failed to functionally substitute for CHO-HS2ST which may indicate a difference in substrate specificity between CHO-HS2ST and Pipe. As part of localization studies, GFP-tagged Pipe was expressed in Cos-7 cells and its subcellular distribution was examined. These studies demonstrated that the activity of Wind (one of the other two somatically required dorsal group genes) is required to direct the localization of GFP-Pipe to the Golgi in Cos-7 cells thus demonstrating a function for Wind.