The Arf-GEF Steppke promotes F-actin accumulation, cell protrusions and tissue sealing during Drosophila dorsal closure

Abstract

Cytohesin Arf-GEFs promote actin polymerization and protrusions of cultured cells, whereas the Drosophila cytohesin, Steppke, antagonizes actomyosin networks in several developmental contexts. To reconcile these findings, we analyzed epidermal leading edge actin networks during Drosophila embryo dorsal closure. Here, Steppke is required for F-actin of the actomyosin cable and for actin-based protrusions. steppke mutant defects in the leading edge actin networks are associated with improper sealing of the dorsal midline, but are distinguishable from effects of myosin mis-regulation. Steppke localizes to leading edge cell-cell junctions with accumulations of the F-actin regulator Enabled emanating from either side. Enabled requires Steppke for full leading edge recruitment, and genetic interaction shows the proteins cooperate for dorsal closure. Inversely, Steppke over-expression induces ectopic, actin-rich, lamellar cell protrusions, an effect dependent on the Arf-GEF activity and PH domain of Steppke, but independent of Steppke recruitment to myosin-rich AJs via its coiled-coil domain. Thus, Steppke promotes actin polymerization and cell protrusions, effects that occur in conjunction with Steppke’s previously reported regulation of myosin contractility during dorsal closure.

SCAR/WAVE complex recruitment to a supracellular actomyosin cable by myosin activators and a junctional Arf-GEF during Drosophila dorsal closure

“…, 2017 ; Zheng et al. , 2019 ), and where Step is specifically required for F-actin accumulation ( West and Harris, 2020 ). Thus, Step seems specifically needed for SCAR/WAVE recruitment and F-actin accumulation where junctional actomyosin levels are high.…”

“…Assembly also involves Rac small G proteins ( Woolner et al. , 2005 ) and the cytohesin Arf-GEF Step ( West and Harris, 2020 ), but involvement of the SCAR/WAVE and Arp2/3 complexes is unknown.…”

Actin polymerization and depolymerization in developing vertebrates

“…Aberrant actin protrusion formation reduces E-cadherin expression and the misalignment of myosin Ⅱ, ultimately affecting adhesion (Ikawa and Sugimura, 2018). Further, disturbing F-actin turnover by gene depletion or acute drug treatments breaks the interaction between the contractile actomyosin network and AJs (Romero et al, 2020;West and Harris, 2020), in which AIP1 and cofilin act as the promoting factors (Romero et al, 2020). This mechanism is important to understand how cells respond to and resist tissue stress during rearrangement.…”