In fruit flies, neurons that release a molecular messenger known as octopamine (green) directly connect to the ovaries (oval shapes, magenta). Image credit: Yoshinari et al. (CC BY 4.0)
Stem cells have the unique ability to mature into the various, specialized groups of cells required for organisms to work properly. Local signals released by the tissues immediately surrounding stem cells usually trigger this specialization process. However, recent studies have revealed that external signals, such as hormones or neurotransmitters (the chemicals used by nerve cells to communicate), can also control the fate of stem cells. This is particularly the case during development, or in response to events such as injury.
In the right conditions, germline stem cells can specialize into the egg or sperm required for many animals to reproduce. In fruit flies for example, the semen contains proteins that activate a cascade of molecular events in the female nervous system, ultimately resulting in female germline stem cells multiplying in the ovaries after mating. Yet, exactly how this process takes place was still unclear. To investigate this question, Yoshinari et al. focused on nerve cells in the fruit fly ovary which produce a neurotransmitter called octopamine.
The experiments assessed changes in the ovaries of female fruit flies after mating, piecing together the sequence of events that activate germline stem cells. This showed that first, mating triggers the release of octopamine from the nerve cells. In turn, this activates a protein called Oamb, which is studded through the membrane of cells present around germline stem cells. Turning on Oamb prompts a cascade of molecular events which include an enzyme called Matrix metalloproteinase 2 regulating the signal sent from the local environment to germline stem cells.
As mammals use a neurotransmitter similar to octopamine, future fruit fly studies could shed light on how neurotransmitters activate stem cells in other animals. Ultimately, unravelling the way external signals trigger the specialization process may offer insight into how diseases arise from uncontrolled stem cell activity.
