ALL IN PROPORTION
Drosophila insulin-like peptides (dILPs) regulate part of the signaling pathway that helps keep organs growing in proportion during development. When the larval fly eats food (1) the fat body detects the spike in dietary nutrients, and in response, fat cells upregulate the TOR signaling pathway (2). The fat body, a single organ that runs the length of the larva, releases a signal that travels to the brain. The nature of the fat-to-brain messenger remains elusive, although recent work implicates a cytokine called UPD2, which has functional similarity to the human hormone leptin. Plentiful nutrients also trigger PI3 kinase signaling in muscle cells (3), likewise sending an unknown signal to the brain that triggers increased feeding behaviors. In response to this information, the brain releases dILP2, dILP3, and dILP5 into the hemolymph (4) to help regulate larval growth. The brain also releases dILP6, which acts locally on brain cells (5) to increase the replication of neural stem cells.
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