Retrograde BMP signaling controls Drosophila behavior through regulation of a peptide hormone battery

Abstract

Retrograde BMP signaling in neurons plays conserved roles in synaptic efficacy and subtype-specific gene expression. However, a role for retrograde BMP signaling in the behavioral output of neuronal networks has not been established. Insect development proceeds through a series of stages punctuated by ecdysis, a complex patterned behavior coordinated by a dedicated neuronal network. In Drosophila, larval ecdysis sheds the old cuticle between larval stages, and pupal ecdysis everts the head and appendages to their adult external position during metamorphosis. Here, we found that mutants of the type II BMP receptor wit exhibited a defect in the timing of larval ecdysis and in the completion of pupal ecdysis. These phenotypes largely recapitulate those previously observed upon ablation of CCAP neurons, an integral subset of the ecdysis neuronal network. Here, we establish that retrograde BMP signaling in only the efferent subset of CCAP neurons (CCAP-ENs) is required to cell-autonomously upregulate expression of the peptide hormones CCAP, Mip and Bursicon β. In wit mutants, restoration of wit exclusively in CCAP neurons significantly rescued peptide hormone expression and ecdysis phenotypes. Moreover, combinatorial restoration of peptide hormone expression in CCAP neurons in wit mutants also significantly rescued wit ecdysis phenotypes. Collectively, our data demonstrate a novel role for retrograde BMP signaling in maintaining the behavioral output of a neuronal network and uncover the underlying cellular and gene regulatory substrates.

Temporally tuned neuronal differentiation supports the functional remodeling of a neuronal network in Drosophila

“…Here, we propose that temporally tuned terminal differentiation provides an alternate mechanism of functional recruitment to an existing neuronal network and show that this mechanism can support a change in network output. The extension of an axon into the periphery and the induction of peptide hormone expression in late CCAP neurons at pupariation are of critical relevance, because these peptide hormones are the primary output of CCAP neurons that direct ecdysis (14,25,38). Together, these data highlight an unanticipated functional heterogeneity in the CCAP neuronal network (24) and outline the regulatory mechanisms and functional relevance of temporally tuned neuronal differentiation in network remodeling.…”

“…The lack of evidence supporting the emergence of late CCAP neurons from postembryonic lineages or early CCAP neuron transfating led us to test whether late CCAP neurons are embryonic but undergo delayed differentiation at pupariation. We previously reported that coexpression of Dac with either pMad or OK6-GAL4 could discriminate T3-A4 CCAP-ENs from all other cells in the dorsal VNC in embryos and larvae (25). Because late CCAP-ENs coexpressed these markers ( Fig.…”

“…Our previous work (25) demonstrated that T3-A4 CCAP-ENs exit the VNC via A1-A5 lateral segmental nerves, and that they are the only axons to express dac GAL4 , UAS-CD8-GFP within those nerve trunks. Here we found that dac GAL4 ,UAS-CD8-GFP-expressing axons are not observed in A6 and A7 lateral segmental nerve trunks in larvae up to mid-L3 (Fig.…”

“…S1 B and B′). Previously, we found that CCAP-ENs can be discriminated from CCAP-INs by their coexpression of Dachshund (Dac), OK6-GAL4, and nuclear phosphorylated Mad (pMad) (25). Nuclear pMad is indicative of active bone morphogenetic protein (BMP) signaling and is exclusive, in the Drosophila VNC, to efferent neurons (26,27).…”

Specification of neuronal subtypes by different levels of Hunchback

“…Distinct CCAPexpressing neurons appear in the T3-A4 segments in the first instar larva; these are efferent neurons (ENs) that exit the ganglion via the lateral segmental nerve and can be identified by the expression of Dachshund (Dac) (Fig. 1C) (Park et al, 2003;Santos et al, 2007;Veverytsa and Allan, 2011;Vomel and Wegener, 2007). Although the ENs of segment T3 always express CCAP in the first instar larva, in A1-4 segments only one or two ENs per hemiganglion usually expresses CCAP at this stage, with expression in the others appearing sequentially during larval development (Fig.…”