Fascin regulates nuclear actin during Drosophila oogenesis

Abstract

Over 40 years of controversy surround the nuclear localization of actin. Now, it is well understood that actin localization to the nucleus is highly regulated, suggesting actin has critical nuclear functions. Studies have demonstrated that nuclear actin has roles in regulating transcription, chromatin remodeling, nuclear organization, and DNA damage repair. However, much remains unknown about nuclear actin, including its structure in the nucleus and its functions during development. Here we present our novel finding that Drosophila oogenesis, or follicle development, is a model for studying the structure, regulation, and function of nuclear actin. Follicles are composed of roughly 1000 somatic follicle cells and 16 germline cells, including 15 nurse or support cells and a single oocyte, and progress through a series of 14 morphological stages, from the germanium to Stage 14. During Stages 5–9, nuclear actin levels are high in the oocyte and exhibit variation within the nurse cells. Known regulators of actin nucleocytoplasmic transport, Cofilin and Profilin, also localize to the nuclei during follicle development. Further, germline expression of GFP‐tagged Actin results in nuclear actin rod formation in nurse cells during Stages 5–9. These nuclear actin rods are static, label with both phalloidin and Cofilin. These findings suggest that nuclear actin is tightly regulated during follicle development. As we have previously determined that Fascin, an actin bundling protein, localizes to nurse cell nuclei, we postulated that Fascin may be a novel regulator of nuclear actin. Overexpression of Fascin enhances the frequency and length of GFP‐Actin rods, and Fascin colocalizes with these rods. Additionally, the loss of Fascin reduces, while the overexpression of Fascin increases, the frequency of nurse cells with high levels of endogenous nuclear actin. However, genetic manipulation of Fascin does not alter the overall nuclear level of actin within the ovary. Thus, these data suggest Fascin regulates the ability of specific cells to accumulate nuclear actin. The loss of Fascin reduces nuclear Cofilin suggesting that Fascin positively regulates nuclear actin through Cofilin, a key factor in the nuclear import of actin. Indeed, Fascin and Cofilin genetically interact, as double heterozygotes exhibit a reduction in the number of nurse cells with high nuclear actin levels. In conclusion, we have established Drosophila oogenesis as a new system to uncover the physiologically important functions and regulation of nuclear actin. Further, we have identified Fascin as a novel regulator of nuclear actin. The nuclear localization of Fascin and Actin is highly conserved among systems, indicating these findings are applicable far beyond Drosophila follicle development.Support or Funding InformationThis project is supported by grants by the National Science Foundation MCB‐1158527, National Institutes of Health R01GM116885 and Carver Trust Medical Research Initiative Grant. D.J.K. is partially supported by NINDS T32NS045549.