All in the timing

Development—the process of building a body, starting from a single cell—is a carefully choreographed dance in which every participant must perform the right actions at the right moments or else the entire progression will go awry. Different groups of cells communicate using protein signals to coordinate their actions, because, if they do not, then, like a dancer leaping before their partner is ready to catch them, they will fall short of their goal—in this case, building a working body.

Researchers in Whitehead Institute Director Ruth Lehmann’s lab wanted to better understand the different groups of cells involved in creating the gonads—the reproductive organs—and in guiding development of the germ cells, the set of cells that become eggs and sperm. In work published in Developmental Cell on June 2, Lehmann lab postdoc Torsten Banisch showed that an unexpected type of cell, the swarm cells, relay a hormone signal to the germ cell precursors, known as primordial germ cells, which prompts them to differentiate and give rise to the eggs. Swarm cells were first identified almost two decades ago, but until now their function has been unknown.

Data from a single cell RNA sequencing analysis of the larval fruit fly Drosophila melanogaster ovary, led by Lehmann lab postdoc Maija Slaidina, revealed which genes are expressed in each cell, information the researchers could then use to figure out what different cell types are doing. Banisch used this data to solve the mystery of how the primordial germ cells know when to differentiate. His findings illuminate an important aspect of how cells precisely coordinate their actions during development. By acting as a signal relay, the swarm cells allow the developing ovary to time germ cell differentiation with the construction of the physical ovary.

Finding the missing messenger

Banisch knew that the initiating event for germ cell differentiation is a burst of the hormone ecdysone. However, primordial germ cells cannot themselves sense ecdysone, so there must be some intermediary cell type, which senses the hormone spike and then relays the cue to the primordial germ cells using a different signal that they can receive.

Banisch first determined that the signal causing the primordial germ cells to differentiate was the protein Torso-like. Then, using the information from the single cell RNA sequencing experiments, Banisch identified the cell type that was releasing Torso-like as the previously mysterious swarm cells. Further experiments uncovered the complete sequence of events: first, all primordial germ cells are kept in a stem-like state, unable to differentiate. Some of the germ cells are sequestered into niches where they remain in a stem-like state in the adult fly, prevented from differentiating by particular localized signals.

The rest of the germ cells, out in the developing gonad, do not receive these signals, but are instead temporarily kept from differentiating by the repressor protein Krüppel. When the time is right, a first ecdysone pulse cues the swarm cells to migrate across the gonads to the posterior. A second ecdysone pulse occurs as the swarm cells pass near the primordial germ cells, and it is then that the swarm cells release the Torso-like protein, which binds to the primordial germ cells and lifts the repression caused by Krüppel, triggering the cells to differentiate and generate the eggs. This carefully timed sequence of events ensures that the germ cells do not differentiate before the ovary is ready for them. Having completed their signaling role, most swarm cells die, while some continue on to their next function: helping to connect the ovary to the rest of the reproductive tract. 

“The timing of when to tell the cells to differentiate is incredibly crucial, because if they differentiate too early, before the structures that deal with them are ready, then you lose the germ line and are effectively infertile,” Banisch says. “What’s exciting about what we found is how these different cell lineages, the germ line and the soma [the cells, including swarm cells, that form the ovary and the rest of the body] communicate in order to coordinate this crucial timing.”
 
Swarm cells are not unique in serving a temporary role as a signaling relay; cells that perform this function are referred to as transitory signaling centers or organizers. Across the animal kingdom such transitory signaling centers coordinate the temporal and physical organization of tissues in the developing embryo. The Lehmann lab’s research demonstrates the importance of transitory signaling centers for enabling cells to coordinate their actions in time-sensitive scenarios.
 
In the course of their work, the researchers identified markers that can be used to tag and manipulate swarm cells, which will allow for future research into their multiple roles in development. Lehmann hopes that, with the tools and RNA sequencing data that the researchers have developed, they will be able to continue filling in the full picture of gonad development.
 
“Single cell sequencing allowed us to take the whole organ apart and use the data to piece together the development of the organ,” Lehmann says. “The ultimate goal is to be able to understand how every cell type in the ovary is set aside initially, what their interactions are, and how they're all working together in time and space to make a functional ovary.”

This work was conducted as the Lehmann lab was moving from the Skirball Institute and HHMI at NYU Langone Medical Center to the Whitehead Institute.