Piwi Is Required in Multiple Cell Types to Control Germline Stem Cell Lineage Development in the Drosophila Ovary

Xing Ma,Haifan Lin,Ying Mao,Yoshiya Nishimoto,William Mcdowell,Trieu Do,Su Wang,Jeff Haug,Mayu Inaba,Hui Li,Allison Peak,Hua Li,Karin Gaudenz,Jungeun Park,Anoja Perera,Yuan Gao,Yukiko Yamashita,Ting Xie,Jian Ni ,Xiu–Peng Song ,Kate E Malanowski ,Lu Ping Liu

doi:10.1371/journal.pone.0090267

Abstract

The piRNA pathway plays an important role in maintaining genome stability in the germ line by silencing transposable elements (TEs) from fly to mammals. As a highly conserved piRNA pathway component, Piwi is widely expressed in both germ cells and somatic cells in the Drosophila ovary and is required for piRNA production in both cell types. In addition to its known role in somatic cap cells to maintain germline stem cells (GSCs), this study has demonstrated that Piwi has novel functions in somatic cells and germ cells of the Drosophila ovary to promote germ cell differentiation. Piwi knockdown in escort cells causes a reduction in escort cell (EC) number and accumulation of undifferentiated germ cells, some of which show active BMP signaling, indicating that Piwi is required to maintain ECs and promote germ cell differentiation. Simultaneous knockdown of dpp, encoding a BMP, in ECs can partially rescue the germ cell differentiation defect, indicating that Piwi is required in ECs to repress dpp. Consistent with its key role in piRNA production, TE transcripts increase significantly and DNA damage is also elevated in the piwi knockdown somatic cells. Germ cell-specific knockdown of piwi surprisingly causes depletion of germ cells before adulthood, suggesting that Piwi might control primordial germ cell maintenance or GSC establishment. Finally, Piwi inactivation in the germ line of the adult ovary leads to gradual GSC loss and germ cell differentiation defects, indicating the intrinsic role of Piwi in adult GSC maintenance and differentiation. This study has revealed new germline requirement of Piwi in controlling GSC maintenance and lineage differentiation as well as its new somatic function in promoting germ cell differentiation. Therefore, Piwi is required in multiple cell types to control GSC lineage development in the Drosophila ovary.

Highlights

Small RNAs have received much attention in recent years because of their important and diverse roles in the regulation of various biological processes [1,2,3,4,5]
Piwi is required in escort cell (EC) to control germ cell differentiation and EC survival To identify the genes that are required in ECs for controlling germ cell differentiation, we carried out a genetic screen using an EC-expressing gal4 driver c587 and transgenic UAS-RNAi lines from the Vienna Drosophila RNAi Center (VDRC)
Piwi was identified for its requirement in ECs to control germ cell differentiation as c587-mediated knockdown of piwi causes the accumulation of spectrosome-containing ill-differentiated single germ cells (SGCs) located distantly from cap cells in the knockdown germaria, which is in great contrast with the control germaria (Fig. 1B)

Summary

Introduction

Small RNAs have received much attention in recent years because of their important and diverse roles in the regulation of various biological processes [1,2,3,4,5]. Recent studies have shown that piRNAs function in somatic cells to regulate gene expression and repress TEs [10,11,12,13,14,15]. Immediate GSC daughters, known as cystoblasts (CBs), move away from cap cells and undergo four rounds of synchronized cell division to form 2-cell, 4-cell, 8-cell and 16-cell cysts. CBs and cysts are tightly encased by cellular processes of escort cells (ECs), known as inner germarial sheath cells (Fig. 1A). Genetic and cell biological studies have shown that terminal filament (TF)/cap cells and anterior ECs form the self-renewing niche for GSCs, which provides the essential BMP signal for repressing GSC differentiation and thereby maintaining their self-renewal [16]

Methods

Results

Conclusion