Neuronal octopamine signaling regulates mating-induced germline stem cell increase in female Drosophila melanogaster

Acceptance summary:

In this manuscript, Niwa and colleagues investigated the mechanism of octopaminergic system in regulating mating-induced GSC increase, a phenomenon previously identified by the same group. A fundamental question remaining in the field is how SPR-positive sensory neurons (SPSNs) relays the mating signal to GSC to increase their numbers after mating. These authors found that Oamb, one of the octopamine (OA) receptors, is expressed in escort cells in the germarium. Using sophisticated genetic tools, they illustrated that OA/Oamb signaling in escort cells is required and sufficient for mating induced GSC increase. OA/Oamb-induced GSC increase depends on Ca²⁺ increase, ecdysone signaling, and Mmp2, similar to the OA-induced follicle rupture in mature follicles. They went on to identify a subset of OA neurons (dsx⁺Tdc2⁺) in the abdominal ganglion responsible for OA signaling in escort cells and GSC increase. Furthermore, they illustrated that SPSNs directly innervate dsx⁺Tdc2⁺ neurons and that acetylcholine released from SPSNs inhibits OA release from dsx⁺Tdc2⁺ neurons, which could be overridden by mating. Therefore, they established a clear relay mechanism from mating to SPSNs, dsx⁺Tdc2⁺ neurons, Oamb signaling in escort cells, and GSC increase.

Overall, all experiments were well designed and included all the controls. The data were interpreted accurately and support their conclusions in general. The work is a nice addition to our understanding of extrinsic cues to stem cell regulation.

Decision letter after peer review:

Thank you for submitting your article "Neuronal octopamine signaling regulates mating-induced germline stem cell proliferation in female Drosophila" for consideration by eLife. Your article has been reviewed by three peer reviewers, including Yukiko M Yamashita as the Reviewing Editor and Reviewer #1, and the evaluation has been overseen by Utpal Banerjee as the Senior Editor. The following individual involved in review of your submission has agreed to reveal their identity: Jianjun Sun (Reviewer #2).

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

We would like to draw your attention to changes in our revision policy that we have made in response to COVID-19 (https://elifesciences.org/articles/57162). Specifically, when editors judge that a submitted work as a whole belongs in eLife but that some conclusions require a modest amount of additional new data, as they do with your paper, we are asking that the manuscript be revised to either limit claims to those supported by data in hand, or to explicitly state that the relevant conclusions require additional supporting data.

Our expectation is that the authors will eventually carry out the additional experiments and report on how they affect the relevant conclusions either in a preprint on bioRxiv or medRxiv, or if appropriate, as a Research Advance in eLife, either of which would be linked to the original paper.

Summary:

In this manuscript, Niwa and colleagues investigated the mechanism of octopaminergic system in regulating mating-induced GSC increase, a phenomenon previously identified by the same group. A fundamental question remaining in the field is how SPR-positive sensory neurons (SPSNs) relays the mating signal to GSC to increase their numbers after mating. These authors found that Oamb, one of the octopamine (OA) receptors, is expressed in escort cells in the germarium. Using sophisticated genetic tools, they illustrated that OA/Oamb signaling in escort cells is required and sufficient for mating induced GSC increase. OA/Oamb-induced GSC increase depends on Ca²⁺ increase, ecdysone signaling, and Mmp2, similar to the OA-induced follicle rupture in mature follicles. They went on to identify a subset of OA neurons (dsx⁺Tdc2⁺) in the abdominal ganglion responsible for OA signaling in escort cells and GSC increase. Furthermore, they illustrated that SPSNs directly innervate dsx⁺Tdc2⁺ neurons and that acetylcholine released from SPSNs inhibits OA release from dsx⁺Tdc2⁺ neurons, which could be overridden by mating. Therefore, they established a clear relay mechanism from mating to SPSNs, dsx⁺Tdc2⁺ neurons, Oamb signaling in escort cells, and GSC increase.

Overall, all experiments were well designed and included all the controls. The data were interpreted accurately and support their conclusions in general. The work is a nice addition to our understanding of extrinsic cues to stem cell regulation. The reviewers identified the following major concerns, which should be addressed in the revision. Given the pandemic, the reviewers feel it to be appropriate if the statements are weakened, acknowledging the caveats in interpretation.

Essential revisions (which can be done with textual changes, unless they do not already have the data):

1) The evidence that the relevant target cells of the octopaminergic neurons are escort cells is rather weak; The authors cannot rule out contributions from follicle cells based on the current data. The authors use drivers expressed in both cell types (C587-Gal4, Tj-gal4) for most knockdown/overexpression experiments. It will be important to exclude this possibility by using a Gal4 driver only expressed in early follicle cells. (R44E10-Gal4 used to exclude other cell types is expressed in follicle cells of stage 14 oocytes---which is too late to assess the involvement of follicle cells). Alternatively, the authors should weaken the statement to be precise on this point.

2) Inferred from Timp experiments, authors conclude that Mmp2 activity in the GSC niche cells is necessary for mating-induced GSC increase (subsection “Matrix metalloproteinase 2 acts downstream of OA-Oamb-Ca²⁺ signaling”, second paragraph). It will be important to examine the Mmp2 expression and activity in both virgin and mated female to see whether Mmp2 activity is indeed regulated by mating. The authors seem to alternate between two models. Throughout most of the manuscript, the mating signal received by the ovary seems to be the sex peptide-regulated octopamine release. However, in the section where the Mmp2/Timp1 data is presented, the implication is that Timp1 expression would go down after mating, resulting in increased Mmp2 activity. Do Timp1 or any other members of the OA-to-Mmp2 signalling axis change after mating? How would the authors reconcile/integrate the two models? Related to this, the epistasis between ecdysone signalling and Mmp2 activity is unclear.