Downregulation of NHP2 promotes proper cyst formation in Drosophila ovary

• Published in: Development, growth & differentiation growth & differentiation, 2018-06, Vol.60 (5), p.248-259
• Main Authors: Morita, Shumpei, Ota, Ryoma, Kobayashi, Satoru
• Format: Article
• Language: English
• Subjects: cyst formation; Cysts; Drosophila; germline; Insects; NHP2; Nhp2 protein; Nucleoli; oogenesis; Ovaries; Phenotypes; Protein biosynthesis; Proteins; Pseudouridylation; rRNA; Stem cells; Transcription
• ISSN: 0012-1592; 1440-169X
• DOI: 10.1111/dgd.12539

In Drosophila ovary, germline stem cells (GSCs) divide to produce two daughter cells. One daughter is maintained as a GSC, whereas the other initiates cyst formation, a process involving four synchronous mitotic divisions that form 2‐, 4‐, 8‐, and 16‐cell cysts. In this study, we found that reduction in the level of NHP2, a component of the H/ACA small nucleolar ribonucleoprotein complex that catalyzes rRNA pseudouridylation, promotes progression to 8‐cell cysts. NHP2 protein was concentrated in the nucleoli of germline cells during cyst formation. NHP2 expression, as well as the nucleolar size, abruptly decreased during progression from 2‐cell to 4‐cell cysts. Reduction in NHP2 activity in the germline caused accumulation of 4‐ and 8‐cell cysts and decreased the number of single cells. In addition, NHP2 knockdown impaired the transition to 16‐cell cysts. Furthermore, a tumorous phenotype caused by Sex‐lethal (Sxl) knockdown, which is characterized by accumulation of single and two‐cell cysts, was partially rescued by NHP2 knockdown. When Sxl and NHP2 activities were concomitantly repressed, the numbers of four‐ and eight‐cell cysts were increased. In addition, Sxl protein physically interacted with NHP2 mRNA in ovaries. Thus, it is reasonable to conclude that Sxl represses NHP2 activity at the post‐transcriptional level to promote proper cyst formation. Because NHP2 knockdown did not affect global protein synthesis in the germarium, we speculate that changes in NHP2‐dependent pseudouridylation, which is involved in translation of specific mRNAs, must be intact in order to promote proper cyst formation. NHP2 protein was concentrated in the nucleoli of germline cells during cyst formation. NHP2 expression, as well as the nucleolar size, abruptly decreased during progression from two‐cell to four‐cell cysts. We found that reduction in the level of NHP2, promotes progression from single cells and two‐cell cysts to four‐ and eight‐cell cysts.