Germ Cell Segregation from the Drosophila Soma Is Controlled by an Inhibitory Threshold Set by the Arf-GEF Steppke

Abstract

Germline cells segregate from the soma to maintain their totipotency, but the cellular mechanisms of this segregation are unclear. The Drosophila melanogaster embryo forms a posterior group of primordial germline cells (PGCs) by their division from the syncytial soma. Extended plasma membrane furrows enclose the PGCs in response to the germ plasm protein Germ cell-less (Gcl) and Rho1-actomyosin activity. Recently, we found that loss of the Arf-GEF Steppke (Step) leads to similar Rho1-dependent plasma membrane extensions but from pseudocleavage furrows of the soma. Here, we report that the loss of step also leads to premature formation of a large cell group at the anterior pole of the embryo . These anterior cells lacked germ plasm, but budded and formed at the same time as posterior PGCs, and then divided asynchronously as PGCs also do. With genetic analyses we found that Step normally activates Arf small G proteins and antagonizes Rho1-actomyosin pathways to inhibit anterior cell formation. A uniform distribution of step mRNA around the one-cell embryo cortex suggested that Step restricts cell formation through a global control mechanism. Thus, we examined the effect of Step on PGC formation at the posterior pole. Reducing Gcl or Rho1 levels decreased PGC numbers, but additional step RNAi restored their numbers. Reciprocally, GFP-Step overexpression induced dosage-and Arf-GEF-dependent loss of PGCs, an effect worsened by reducing Gcl or actomyosin pathway activity. We propose that a global distribution of Step normally sets an inhibitory threshold for Rho1 activity to restrict early cell formation to the posterior.

Germ Cell-less Promotes Centrosome Segregation to Induce Germ Cell Formation

“…PGC cellularization requires the concerted action of many proteins that regulate the organization of the actin cytoskeleton, including Anillin, to mediate closure of the PB furrow (Field et al, 2005). Recent studies suggest that gcl could play a role in controlling the reorganization of the actin cytoskeleton during PB furrow closure via Anillin (Cinalli and Lehmann, 2013) and/or Steppke (Step), a recently identified antagonist of Rho1 (Lee et al, 2015). Loss of Step activity results in precocious cellularization in the soma, while overexpressing Step blocks PGC formation by a mechanism that involves antagonizing Gcl activity (Lee et al, 2015).…”

GCL and CUL3 Control the Switch between Cell Lineages by Mediating Localized Degradation of an RTK

“…Instead, additional downstream signaling pathways may require ligand-induced Torso receptor activation. Candidates include the JAK/STAT pathway, which is activated in dominant gain-of-function alleles of Torso (Li et al, 2002), and the ARF-GEF Steppke, which has been recently identified as a negative regulator of PGC formation (Lee et al, 2015). It will be of interest to explore further whether and how these alternative pathways oppose PGC formation.…”

PH Domain-Arf G Protein Interactions Localize the Arf-GEF Steppke for Cleavage Furrow Regulation in Drosophila

“…Drosophila embryogenesis begins as a syncytium in which PM cleavage furrows transiently separate dividing somatic nuclei and then fully cellularize ~6000 nuclei to form the cellular blastoderm [ 11 – 13 ]. Step localizes to these cleavage furrows and uses its Arf-GEF activity to antagonize the Rho1-actomyosin pathway during two forms of cell division [ 14 , 15 ]. Step acts first to control primordial germ cell division from the syncytial soma, and without Step, the division occurs at ectopic sites [ 15 ].…”

“…Step localizes to these cleavage furrows and uses its Arf-GEF activity to antagonize the Rho1-actomyosin pathway during two forms of cell division [ 14 , 15 ]. Step acts first to control primordial germ cell division from the syncytial soma, and without Step, the division occurs at ectopic sites [ 15 ]. Then, Step continues to act during the syncytial nuclear divisions of the soma to prevent closure of cleavage furrows at their basal tips.…”