Drosophila DCAF12 Functions in a Negative Feedback Loop to Regulate Synaptic BMP Signaling

Abstract

Synaptic growth at the Drosophila neuromuscular junction is regulated by a canonical retrograde Transforming Growth Factor‐β (TGF‐β)/Bone Morphogenetic Protein (BMP) signaling pathway. Binding of the canonical BMP ligand, Glass bottom boat (Gbb), to a presynaptic BMP receptor complex on the motor neuron leads to phosphorylation of the transcription factor, Mothers against decapentaplegic (Mad). Phosphorylated Mad (pMad) associates with a cofactor, Medea (Med), and enters the nucleus where it regulates the expression of genes that promote synaptic growth. This retrograde signaling is necessary for normal synaptic growth (McCabe et al. 2003). Additionally, presynaptic, autocrine Gbb has been shown to mediate evoked and spontaneous release and homeostatic plasticity, but the precise mechanisms remain unclear. Previously, we showed that Drosophila DCAF12 is required for multiple independent synaptic functions at the larval NMJ, including the upregulation of evoked neurotransmitter release and the downregulation of glutamate receptor subunits. Here, we show that presynaptic DCAF12 is a negative regulator of TGF‐β signaling at larval NMJs, and that DCAF12 is part of a negative feedback loop regulating TGF‐β signaling at the synapse. Presynaptic DCAF12 regulates synaptic pMad: deletions of DCAF12 increase synaptic pMad at larval NMJs, while transgenically expressing DCAF12 in neurons at these mutant NMJs restores pMad to control levels at the synapse. The increase in synaptic pMad seen at dcaf12 deletion NMJs can be replicated with neuronal overexpression (OE) of the BMP ligand Gbb, or its type II BMP receptor, Wishful thinking (Wit). Additionally, OE of Wit in dcaf12 ‐/‐ NMJs induces a novel overgrowth phenotype not seen with Wit OE alone. We propose that DCAF12 controls the output of the BMP type II receptor Wishful thinking by regulating its internalization or trafficking at the Drosophila NMJ. In turn, presynaptic DCAF12 levels are regulated by retrograde BMP signaling, with BMP signaling knockdowns producing decreases in presynaptic DCAF12.