Planer-polarized junction association of the RacGEF ELMO-Sponge contributes to planar polarity and germband extension in Drosophila embryos.

Planar polarization of epithelia is critical for tissue morphogenesis and function. Polarized cell behaviour requires the asymmetric distribution of key components. The planar polarized enrichment of F-actin and MyoII is required for Drosophila embryonic axis extension. The actomyosin enriches the cell junctions in the dorsoventral (vertical) versus anterior-posterior (horizontal) direction to shrink the vertical junctions and drive cell rearrangement. Rac GEF plays an essential role in the reorganization of the actin cytoskeleton in many cellular processes. However, whether and how Rac GEF is involved in the planar polarity of the cytoskeleton in Drosophila embryonic axis extension remains largely unclear. Here, we report that the planar polarized RacGEF ELMO-Sponge is essential for germband extension. We find that Sponge and ELMO form a complex and enrich the vertical rather than the horizontal junctions in germband cells. Critically, this planar polarized distribution is confined to the region of adherens junctions in the apical-basal dimension, where F-actin becomes concentrated. Both the amount and planar polarity of F-actin, MyoII, and E-cadherin were affected by ELMO knockdown. Sponge knockdown damaged tissue integrity during germband extension. Consistent with reduced MyoII levels, we observed reduced recoil after junction ablation and incomplete germband extension in sponge- or ELMO-depleted embryos. Taken together, our data suggest that the ELMO/Sponge complex is required for the planar polarized localization of F-actin and maintains junctional contractility, thereby promoting effective tissue elongation.