Abstract:
The syncytial nuclear division cycles during early Drosophila embryo development involves
orchestrated remodelling of the membrane and cytoskeleton. We have shown that the F-BAR
domain protein Syndapin, that is known to have roles in membrane tubulation, endocytosis
and cytoskeletal remodelling, is increasingly enriched at the tip of syncytial pseudocleavage
furrows with each nuclear cycle. The dynamics of pseudocleavage furrow progression
depends on Syndapin function. We showed that Syndapin function was important for
recruitment of septin Peanut and organization of the formin Diaphanous at the furrow tip
downstream of the furrow initiation protein RhoGEF2. Syndapin-depleted embryos showed
disorganized actin on the furrow, also seen in RhoGEF2 mutants. Interstingly, Syndapin
overexpression in RhoGEF2-depleted background, restored actin organization and extent of
furrow ingression. Addtionally, we showed that Syndapin or RhoGEF2 depletion causes
reduction in the number of Rab5 puncta; however, Syndapin overexpression in RhoGEF2-
depleted background cannot restore the number of Rab5 puncta. Therefore, we conclude that
Syndapin can rescue the furrow extension phenotype by actin organization alone and not
because of its role in early endocytosis.
Actin turnover is also crucial for dynamic expansion of the apical caps prior to furrow
formation. Arp2/3 function is important for expansion of the apical cap. We find that
syndapin mutants limit cap expansion and show abnormally large actin caps. We are able to
rescue this cap expansion phenotype by combining mutants of a component of the Arp2/3
complex, ArpC1 with the syndapin mutant. Using TIRF microscopy, we find that
remodelling of apical actin structures is defective in these mutants giving rise to the observed
phenotypes. Therefore, Syndapin and Arp2/3 complex oppose each other to regulate
dynamics of actin cap expansion in the syncytial embryo.
Apical cap expansion defects also correlate with defects in furrow extension and formation of
edges of polygonal compartments where the expanding caps meet. Therefore, we also studied
effects of Syndapin depletion and also of other actin regulatory proteins on the distribution of
polygon classes that are conserved across organized epithelial sheets in various tissues with a
maximum of hexagons. The result was a frequency distribution skewed towards pentagons
rather than hexagons. Interestingly, however, the recruitment of junctional polarity proteins
was not affected in embryos depleted of Syndapin.
