Single serotonergic neurons that modulate aggression in Drosophila.

Monoamine serotonin (5HT) has been linked to aggression for many years across species. However, elaboration of the neurochemical pathways that govern aggression has proven difficult because monoaminergic neurons also regulate other behaviors. There are approximately 100 serotonergic neurons in the Drosophila nervous system, and they influence sleep, circadian rhythms, memory, and courtship. In the Drosophila model of aggression, the acute shut down of the entire serotonergic system yields flies that fight less, whereas induced activation of 5HT neurons promotes aggression. Using intersectional genetics, we restricted the population of 5HT neurons that can be reproducibly manipulated to identify those that modulate aggression. Although similar approaches were used recently to find aggression-modulating dopaminergic and Fru(M)-positive peptidergic neurons, the downstream anatomical targets of the neurons that make up aggression-controlling circuits remain poorly understood. Here, we identified a symmetrical pair of serotonergic PLP neurons that are necessary for the proper escalation of aggression. Silencing these neurons reduced aggression in male flies, and activating them increased aggression in male flies. GFP reconstitution across synaptic partners (GRASP) analyses suggest that 5HT-PLP neurons form contacts with 5HT1A receptor-expressing neurons in two distinct anatomical regions of the brain. Activation of these 5HT1A receptor-expressing neurons, in turn, caused reductions in aggression. Our studies, therefore, suggest that aggression may be held in check, at least in part, by inhibitory input from 5HT1A receptor-bearing neurons, which can be released by activation of the 5HT-PLP neurons.

Src tyrosine kinase signaling antagonizes nuclear localization of FOXO and inhibits its transcription factor activity.

Biochemical experiments in mammalian cells have linked Src family kinase activity to the insulin signaling pathway. To explore the physiological link between Src and a central insulin pathway effector, we investigated the effect of different Src signaling levels on the Drosophila transcription factor dFOXO in vivo. Ectopic activation of Src42A in the starved larval fatbody was sufficient to drive dFOXO out of the nucleus. When Src signaling levels were lowered by means of loss-of-function mutations or pharmacological inhibition, dFOXO localization was shifted to the nucleus in growing animals, and transcription of the dFOXO target genes d4E-BP and dInR was induced. dFOXO loss-of-function mutations rescued the induction of dFOXO target gene expression and the body size reduction of Src42A mutant larvae, establishing dFOXO as a critical downstream effector of Src signaling. Furthermore, we provide evidence that the regulation of FOXO transcription factors by Src is evolutionarily conserved in mammalian cells.