Cyclin E controlsDrosophilafemale germline stem cell maintenance independently of its role in proliferation by modulating responsiveness to niche signals

Abstract

SUMMARYStem cells must proliferate while maintaining 'stemness'; however, much remains to be learned about how factors that control the division of stem cells influence their identity. Multiple stem cell types display cell cycles with short G1 phases, thought to minimize susceptibility to differentiation factors. Drosophila female germline stem cells (GSCs) have short G1 and long G2 phases, and dietdependent systemic factors often modulate G2. We previously observed that Cyclin E (CycE), a known G1/S regulator, is atypically expressed in GSCs during G2/M; however, it remained unclear whether CycE has cell cycle-independent roles in GSCs or whether it acts exclusively by modulating the cell cycle. In this study, we detected CycE activity during G2/M, reflecting its altered expression pattern, and showed that CycE and its canonical partner, Cyclin-dependent kinase 2 (Cdk2), are required not only for GSC proliferation, but also for GSC maintenance. In genetic mosaics, CycE-and Cdk2-deficient GSCs are rapidly lost from the niche, remain arrested in a G1-like state, and undergo excessive growth and incomplete differentiation. However, we found that CycE controls GSC maintenance independently of its role in the cell cycle; GSCs harboring specific hypomorphic CycE mutations are not efficiently maintained despite normal proliferation rates. Finally, CycE-deficient GSCs have an impaired response to niche bone morphogenetic protein signals that are required for GSC self-renewal, suggesting that CycE modulates niche-GSC communication. Taken together, these results show unequivocally that the roles of CycE/Cdk2 in GSC division cycle regulation and GSC maintenance are separable, and thus potentially involve distinct sets of phosphorylation targets.

Co-expressed Cyclin D variants cooperate to regulate proliferation of germline nuclei in a syncytium

“…In contrast to regulation of cycling of invertebrate C. elegans and Drosophila premeiotic GSCs, where a G1 is nearly absent and Cyclin E is expressed throughout the cycle, 18,21 we show that O. dioica, pre-meiotic, germline nuclei have a G1-phase. During the period of syncytial germline nuclear proliferation, both the Cyclin Db and Dd variants are expressed.…”

“…18,19,21 The O. dioica Cyclin D variants, Da, Db, Dc and Dd are encoded by 4 different genes. 24 Previously, we have shown that O. dioica Cyclin Dd regulates G1-phase in mitotically proliferating cells during early development and in post-metamorphic somatic endocycling cells.…”

“…[18][19][20][21] In this context, cell cycle profiles of Cyclin E-CDK activity are altered with Cyclin E present throughout the GSC cell cycle. 18,21 In these invertebrates, proliferative renewal of GSCs is nonetheless still responsive to insulin and TOR signaling 2,22 but these nutrient-dependent controls operate during G2, as opposed to G1. In contrast, mouse spermatogonial stem cells proliferate with a distinct G1 phase and Cyclin D plays a major role in the renewal of stem cells.…”

Bam and Otu can regulate stem cell fate by stabilizing cyclin A

“…Therefore, the three B-type cyclins work in specific ways to maintain the balance between GSC self-renewal and differentiation. The G1/S cyclin E is also essential for the maintenance of female GSCs (18). Taken together, these data show that the levels of CycA and several other cyclins are critical to ensure the proper balance between self-renewal and differentiation in stem cells.…”

“…Because cells are more responsive to extracellular cues in G1, a short G1 would favor selfrenewal by shortening the time window during which stem cells can respond to differentiation signals. In Drosophila female GSCs, the G1 phase is also short and most of the cell cycle is spent in G2 (18). In addition, recent work in human embryonic stem cells shows that the maintenance of their pluripotency is actively controlled in S and G2 phases, through the ATM and Cyclin B1 pathways (20).…”