MIPP1 functions at the basolateral domain to facilitate the generation of filopodia and the extension of lamellopodia of tracheal leading cells. Yim Ling Cheng, Deborah Andrew. Cell Biology, Johns Hopkins School of Medicine, Baltimore, MD.

   Multiple Inositol Polyphosphate Phosphatase 1, MIPP1, is a histidine phosphatase that dephosphorylates higher order inositol polyphosphates (InsP8 to InsP4). MIPP is highly conserved, but its biological function is unknown. mipp1 was identified in our lab as a target of Trachealess, the major transcription factor regulating tracheal development. mipp1 is highly expressed in all tracheal cells at early stages and the expression is maintained in only the intercalated branches (e.g. the dorsal branches), with enhanced expression in the branch tips at later stages. We generated a knockout allele of mipp1 and observed that 40% of dorsal branches have delayed sister cell intercalation (SCI), which is the process whereby tracheal tubes elongate by rearranging the cells from a side-by-side to an end-to-end configuration. C-terminal GFP-tagged fly MIPP1 localizes to the ER, consistent with localization of the mammalian protein; however, immunostaining with our recently generated MIPP1 antibodies shows that MIPP1 localizes to the plasma membrane. Topology/structure predictions and biochemical assays, including glycosidase treatment and trypsin protease digestion, reveal that most of the MIPP1 protein, including the active phosphatase domain, faces outside of the cell, with either a C-terminal transmembrane domain or a GPI-link. At early stages, MIPP1 localizes to both apical and basolateral surfaces. During branching morphogenesis, apical levels of MIPP1 decline and the basolateral levels associated with filopodia/lamellopodia increase. Overexpression of MIPP1 increases the number of filopodia and extension of lamellapodia in the leading cells. The pulling force that stimulates SCI is induced by FGF signaling in the tracheal branch leading cells. We find that disruption of FGF signaling results in MIPP1 localizing to the apical surface even in the late stages. We propose that FGF signaling causes MIPP1 to preferentially localize to the basolateral domain where it facilitates the filopodial formation and lamellopodial extension, thus contributing to the pulling forces that underlie SCI.