ABSTRACT
Highwire (Hiw), a putative RING finger E3 ubiquitin ligase, negatively regulates synaptic growth at the neuromuscular junction (NMJ) in Drosophila. hiw mutants have dramatically larger synaptic size and increased numbers of synaptic boutons. Here we show that Hiw binds to the Smad protein Medea (Med). Med is part of a presynaptic bone morphogenetic protein (BMP) signaling cascade consisting of three receptor subunits, Wit, Tkv, and Sax, in addition to the Smad transcription factor Mad. When compared to wild-type, mutants of BMP signaling components have smaller NMJ size, reduced neurotransmitter release, and aberrant synaptic ultrastructure. BMP signaling mutants suppress the excessive synaptic growth in hiw mutants. Activation of BMP signaling, which in wild-type does not cause additional growth, in hiw mutants does lead to further synaptic expansion. These results reveal a balance between positive BMP signaling and negative regulation by Highwire, governing the growth of neuromuscular synapses.
Subject(s)
Bone Morphogenetic Proteins/metabolism , Cell Differentiation/genetics , Drosophila Proteins/metabolism , Drosophila melanogaster/growth & development , Nerve Tissue Proteins/metabolism , Neuromuscular Junction/growth & development , Animals , Bone Morphogenetic Proteins/genetics , Cell Size/genetics , DNA-Binding Proteins/genetics , DNA-Binding Proteins/metabolism , Drosophila Proteins/genetics , Drosophila melanogaster/metabolism , Drosophila melanogaster/ultrastructure , Microscopy, Electron , Motor Neurons/cytology , Motor Neurons/metabolism , Mutation/genetics , Nerve Tissue Proteins/genetics , Neuromuscular Junction/metabolism , Neuromuscular Junction/ultrastructure , Presynaptic Terminals/metabolism , Presynaptic Terminals/pathology , Presynaptic Terminals/ultrastructure , Protein Binding/genetics , Receptors, Cell Surface/genetics , Receptors, Cell Surface/metabolism , Receptors, Transforming Growth Factor beta/genetics , Receptors, Transforming Growth Factor beta/metabolism , Signal Transduction/genetics , Smad4 Protein , Synaptic Transmission/genetics , Trans-Activators/genetics , Trans-Activators/metabolismABSTRACT
Amidated neuropeptides of the FMRFamide class regulate numerous physiological processes including synaptic efficacy at the Drosophila neuromuscular junction (NMJ). We demonstrate here that mutations in wishful thinking (wit) a gene encoding a Drosophila Bmp type 2 receptor that is required for proper neurotransmitter release at the neuromuscular junction, also eliminates expression of FMRFa in that subset of neuroendocrine cells (Tv neurons) which provide the systemic supply of FMRFa peptides. We show that Gbb, a Bmp ligand expressed in the neurohemal organ provides a retrograde signal that helps specify the peptidergic phenotype of the Tv neurons. Finally, we show that supplying FMRFa in neurosecretory cells partially rescues the wit lethal phenotype without rescuing the primary morphological or electrophysiological defects of wit mutants. We propose that Wit and Gbb globally regulate NMJ function by controlling both the growth and transmitter release properties of the synapse as well as the expression of systemic modulators of NMJ synaptic activity.
Subject(s)
Drosophila Proteins/metabolism , Drosophila/metabolism , FMRFamide/genetics , Gene Expression Regulation, Developmental , Receptors, Cell Surface/metabolism , Transforming Growth Factor beta/metabolism , Animals , FMRFamide/metabolism , Neuromuscular Junction/metabolism , Signal Transduction/physiologyABSTRACT
We show that the BMP ortholog Gbb can signal by a retrograde mechanism to regulate synapse growth of the Drosophila neuromuscular junction (NMJ). gbb mutants have a reduced NMJ synapse size, decreased neurotransmitter release, and aberrant presynaptic ultrastructure. These defects are similar to those we observe in mutants of BMP receptors and Smad transcription factors. However, whereas these BMP receptors and signaling components are required in the presynaptic motoneuron, Gbb expression is required in large part in postsynaptic muscles; gbb expression in muscle rescues key aspects of the gbb mutant phenotype. Consistent with this notion, we find that blocking retrograde axonal transport by overexpression of dominant-negative p150/Glued in neurons inhibits BMP signaling in motoneurons. These experiments reveal that a muscle-derived BMP retrograde signal participates in coordinating neuromuscular synapse development and growth.
