ABSTRACT
Retrograde signaling is essential for coordinating the growth of synaptic structures; however, it is not clear how it can lead to modulation of cytoskeletal dynamics and structural changes at presynaptic terminals. We show that loss of retrograde bone morphogenic protein (BMP) signaling at the Drosophila larval neuromuscular junction (NMJ) leads to a significant reduction in levels of Rac GEF Trio and a diminution of transcription at the trio locus. We further find that Trio is required in motor neurons for normal structural growth. Finally, we show that transgenic expression of Trio in motor neurons can partially restore NMJ defects in larvae mutant for BMP signaling. Based on our findings, we propose a model in which a retrograde BMP signal from the muscle modulates GTPase activity through transcriptional regulation of Rac GEF trio, thereby regulating the homeostasis of synaptic growth at the NMJ.
Subject(s)
Bone Morphogenetic Proteins/physiology , Drosophila Proteins/biosynthesis , Guanine Nucleotide Exchange Factors/biosynthesis , Motor Neurons/physiology , Neuromuscular Junction/physiology , Phosphoproteins/biosynthesis , Protein Serine-Threonine Kinases/biosynthesis , Synapses/physiology , Animals , Cell Line , Drosophila , Gene Expression Regulation, Developmental , Humans , Signal Transduction/physiology , Synapses/ultrastructureABSTRACT
At the Drosophila melanogaster larval neuromuscular junction (NMJ), a motor neuron releases glutamate from 30-100 boutons onto the muscle it innervates. How transmission strength is distributed among the boutons of the NMJ is unknown. To address this, we created synapcam, a version of the Ca2+ reporter Cameleon. Synapcam localizes to the postsynaptic terminal and selectively reports Ca2+ influx through glutamate receptors (GluRs) with single-impulse and single-bouton resolution. GluR-based Ca2+ signals were uniform within a given connection (that is, a given bouton/postsynaptic terminal pair) but differed considerably among connections of an NMJ. A steep gradient of transmission strength was observed along axonal branches, from weak proximal connections to strong distal ones. Presynaptic imaging showed a matching axonal gradient, with higher Ca2+ influx and exocytosis at distal boutons. The results suggest that transmission strength is mainly determined presynaptically at the level of individual boutons, possibly by one or more factors existing in a gradient.
Subject(s)
Axons/physiology , Larva/physiology , Motor Neurons/cytology , Neuromuscular Junction/cytology , Synaptic Transmission/physiology , Animals , Animals, Genetically Modified , Calcium Signaling/physiology , Calcium Signaling/radiation effects , Diagnostic Imaging/methods , Drosophila , Drosophila Proteins/deficiency , Drosophila Proteins/genetics , Electric Stimulation/methods , Evoked Potentials/physiology , Evoked Potentials/radiation effects , Gene Expression Regulation, Developmental/physiology , Immunohistochemistry/methods , Luminescent Proteins/metabolism , Membrane Potentials/physiology , Mutagenesis, Insertional/physiology , Neuromuscular Junction/physiology , Neuromuscular Junction/radiation effects , Patch-Clamp Techniques/methods , Synaptic Transmission/radiation effectsABSTRACT
Presynaptic I(h) channels become activated during a tetanus through membrane hyperpolarization resulting from Na(+) accumulation and electrogenic Na(+)/K(+) exchange. I(h) activation is obligatory for inducing long-term facilitation (LTF), a long-lasting synaptic strengthening. cAMP-induced synaptic enhancement also requires I(h) activation, and both processes are sensitive to actin depolymerization. Other mechanisms are responsible for expression of the responses. Once initiated, continued response to cAMP is I(h) and actin independent. Moreover, LTF-induced activation of I(h) renders subsequent cAMP enhancement insensitive to both I(h) blockers and actin depolymerization. This actin-stabilized "temporal synaptic tagging" set by I(h) activation is prolonged when I(h) is activated concurrent with an elevation in presynaptic calcium concentration ([Ca(2+)]i), permitting the further strengthening of synapses given appropriate additional stimuli.
