Insulin-producing cells (magenta) and their neighbours, DH44-producing cells (green), in the fruit fly brain. These cell types sense sugar in different ways and regulate the fly’s metabolism and behavior. Image Credit: Rituja Bisen (CC BY 4.0)
Most animals, including humans, use a hormone called insulin to regulate their blood sugar levels and the balance of energy in their bodies. In fruit flies, insulin is released into the hemolymph – the liquid that acts as blood in insects – by neurons that sit directly on top of the brain. These ‘insulin-producing cells’, or IPCs, play the same role as the beta cells that create insulin in the human pancreas. As fruit flies can easily be genetically manipulated, IPCs have drawn increased interest as a model system to investigate insulin regulation. However, exactly how these cells behave in live insects has remained poorly understood.
To address this knowledge gap, Bisen et al. used an approach that allowed them to record the activity of individual IPCs in live fruit flies under different conditions. The results showed that IPCs release insulin when sugars are eaten as part of a meal, but not when these molecules are directly introduced into the insects via injection. In humans, this well-known phenomenon is known as the incretin effect; it points to insulin release being controlled by complex mechanisms involving gut hormones from the digestive system rather than by a simple increase in blood sugar levels. Bisen et al. also found that the activity of IPCs was much lower in older flies, which may indicate changes in how the insects process sugars later in life.
Whether insects ‘decide’ to look for food is closely tied to variations in their energy reserves, which are linked to insulin release. Additional experiments during which IPCs were artificially stimulated (therefore replicating what would normally happen following a meal and an increase in circulating sugar levels) showed that these cells only play a minor role in modulating food searching behavior compared to other neurons such as DH44-producing cells.
Taken together, these results refine our understanding of the circuits that control insulin release in fruit flies, allowing further examinations that could lead to insights relevant to human health and diseases, such as diabetes.
