Luo et al. describe how the Drosophila ovarian niche limits its own ability to maintain germline stem cells (GSCs) in an undifferentiated state.
Stem cells reside in specialized microenvironments, or niches, that prevent them from differentiating prematurely. In the germaria of Drosophila ovaries, GSCs reside next to cap cells that inhibit differentiation by secreting the TGFβ-like molecule Dpp. In other tissues, Dpp can influence cell fate over long distances, but, in the ovary, its range is spatially restricted such that, when a GSC divides, one of its daughters is positioned too far away from the cap cells to receive the Dpp signal, and therefore starts to differentiate into an egg. Luo et al. were interested in how escort cells—another component of the ovarian niche—help to limit Dpp’s sphere of influence.
The researchers found that escort cells express the Dpp receptor Tkv so that they can mop up excess Dpp secreted from the cap cells. Depleting Tkv from the escort cells extended Dpp’s range, resulting in an increased number of undifferentiated germ cells. Tkv’s expression in escort cells was stimulated by several Wnt ligands released from the cap cells. The ovarian niche therefore limits its own ability to maintain GSCs, ensuring that the stem cells’ progeny can undergo differentiation.
Tkv’s function in escort cells does not require canonical Dpp signaling. Senior author Yu Cai now wants to investigate whether the receptor activates noncanonical signaling pathways instead.
Text by Ben Short