A brain circuit that synchronizes growth and maturation revealed through Dilp8 binding to Lgr3

Brain keeps body size and shape in check Animal systems show amazing left-right symmetry—think of how our legs or arms, or the legs or wings of an insect, are matched in size and shape. Environmental insults and growth defects can challenge these developmental programs. In order to limit the resultant variation, juvenile organisms buffer variability through homeostatic mechanisms, so that the correct final size is attained. Vallejo et al. report that the Drosophila brain mediates such homeostatic control via an insulin-like peptide Dilp8 binding to the relaxin hormone receptor Lgr3. Lgr3 neurons distribute this information to other neuronal populations to adjust the hormones ecdysone, insulin, and juvenile hormone in a manner that stabilizes body and organ size. Science, this issue p. 10.1126/science.aac6767 Drosophila Lgr3 defines a neural circuit for homeostatic regulation of body size. INTRODUCTION Animals have a remarkable capacity to maintain a constant size, even in the face of genetic and environmental perturbations. Size imperfections and asymmetries have an effect on fitness, potentially decreasing competitiveness, survival, and reproductive success. Therefore, immature animals must employ homeostatic mechanisms to counteract substantial size variations and withstand developmental growth perturbations caused by genetic errors, disease, environmental factors, or injury. Such mechanisms ensure that, despite inevitable variations, the appropriate final body size is attained. A better understanding of homeostatic size maintenance will afford insights into normal organ and organismal size control, as well as the developmental origin of anomalous random left-right asymmetries. RATIONALE The Drosophila insulin-like peptide Dilp8 has been shown to mediate homeostatic regulation. When growth is disturbed, Dilp8 is strongly activated and sexual maturation is postponed until the affected elements are recomposed; simultaneously, the growth of other organs is retarded during this process. This compensatory mechanism allows the growth of the affected tissues to catch up. It maintains the synchrony between organs so that the animals achieve the correct size, preserving proportionality and bilateral symmetry. However, the Dilp8 receptor and its site of action remain uncharacterized. RESULTS We found that Dilp8 binds to andactivates the relaxin leucine-rich repeat–containing G protein–coupled receptor Lgr3 to mediate homeostatic control through a pathway dependent on adenosine 3′,5′-monophosphate. Larvae that lack lgr3 in neurons alone do not respond to Dilp8, indicating that the homeostatic system is centered in the brain. Dilp8 delays reproductive maturation by suppressing the neurons releasing the prothoracicotropic hormone (PTTH), which projects to the prothoracic gland and regulates ecdysone production for growth termination. However, this modulation alone is insufficient to adjust growth and stabilize body size. We show that Dilp8-Lgr3 balances growth against the extended growth period by dampening the production of dilp3 and dilp5 by insulin-producing cells (IPCs) in the brain and inhibiting synthesis of the juvenile hormone (JH). We also identify two pairs of dorsomedial neurons in the pars intercerebralis that are necessary and sufficient to mediate the effects of Dilp8. Simultaneous detection of pre- and postsynaptic markers revealed that the Lgr3 neurons mediating this homeostatic control have extensive axonal arborizations. Genetic and GRASP (GFP reconstitution across synaptic partners) analyses demonstrate that these neurons are connected to both the IPCs and PTTH neurons critical for adjusting growth and maturation rate, respectively. Thus, through their extensive axonal arborizations, Lgr3 neurons function like a “neuronal hub”: They route peripheral information about growth status to other neuronal populations, thereby synchronizing damaged tissues and other (undamaged) ones and allocating additional development time so that each organ attains the correct size and maintains proportionality and symmetry. CONCLUSION We identified the relaxin receptor Lgr3 as a Dilp8 receptor and defined a brain circuit for homeostatic control of organismal and organ size in the face of perturbations. Lgr3 neurons that respond to Dilp8 signals directly input on the insulin-producing cells and the PTTH-producing neurons. As Lgr3 outputs, the modulation of these neuronal populations according to Dilp8 levels is critical to delay maturation and promote growth compensation in a manner that stabilizes body size. Without adequate Dilp8-Lgr3 signaling, the brain is incapable of stabilizing size between the distinct body parts, and we see left-right asymmetries and size variations that are greater than usual, reflecting developmental instability. Dilp8-Lgr3 neural circuit and outputs for body-size homeostasis. The brain detects growth status and anomalies via Dilp8 activation of the Lgr3 receptor in a pair of symmetric neurons. These neurons distribute this information to IPCs and PTTH neurons, which then trigger the hormonal responses that regulate size. Without Dilp8-Lgr3 homeostasis, the brain cannot correct variation, and identical body parts can display imperfect symmetry and size. Body-size constancy and symmetry are signs of developmental stability. Yet, it is unclear exactly how developing animals buffer size variation. Drosophila insulin-like peptide Dilp8 is responsive to growth perturbations and controls homeostatic mechanisms that coordinately adjust growth and maturation to maintain size within the normal range. Here we show that Lgr3 is a Dilp8 receptor. Through the use of functional and adenosine 3′,5′-monophosphate assays, we defined a pair of Lgr3 neurons that mediate homeostatic regulation. These neurons have extensive axonal arborizations, and genetic and green fluorescent protein reconstitution across synaptic partners show that these neurons connect with the insulin-producing cells and prothoracicotropic hormone–producing neurons to attenuate growth and maturation. This previously unrecognized circuit suggests how growth and maturation rate are matched and co-regulated according to Dilp8 signals to stabilize organismal size.