Newly Identified Electrically Coupled Neurons Support Development of theDrosophilaGiant Fiber Model Circuit

Abstract

The Drosophila giant fiber (GF) escape circuit is an extensively studied model for neuron connectivity and function. Researchers have long taken advantage of the simple linear neuronal pathway, which begins at peripheral sensory modalities, travels through the central GF interneuron (GFI) to motor neurons, and terminates on wing/leg muscles. This circuit is more complex than it seems, however, as there exists a complex web of coupled neurons connected to the GFI that widely innervates the thoracic ganglion. Here, we define four new neuron clusters dye coupled to the central GFI, which we name GF coupled (GFC) 1–4. We identify new transgenic Gal4 drivers that express specifically in these neurons, and map both neuronal architecture and synaptic polarity. GFC1–4 share a central site of GFI connectivity, the inframedial bridge, where the neurons each form electrical synapses. Targeted apoptotic ablation of GFC1 reveals a key role for the proper development of the GF circuit, including the maintenance of GFI connectivity with upstream and downstream synaptic partners. GFC1 ablation frequently results in the loss of one GFI, which is always compensated for by contralateral innervation from a branch of the persisting GFI axon. Overall, this work reveals extensively coupled interconnectivity within the GF circuit, and the requirement of coupled neurons for circuit development. Identification of this large population of electrically coupled neurons in this classic model, and the ability to genetically manipulate these electrically synapsed neurons, expands the GF system capabilities for the nuanced, sophisticated circuit dissection necessary for deeper investigations into brain formation.

Genetic background mutations drive neural circuit hyperconnectivity in a fragile X syndrome model

“…Alternatively, inappropriate partners could be solely linked by chemical synapses. The identified targets of the excess synaptic projections are the GFC2 and GFC3 neurons, newly established giant fiber circuit members [29]. GFC2/3 neurons contact the GFI with en passant synapses at both the more proximal inframedial bridge and more distal axonal bends.…”

“…GFC2/3 neurons contact the GFI with en passant synapses at both the more proximal inframedial bridge and more distal axonal bends. GFC2/3 neurons have quite complex architectures, and it is unclear if the GFI synaptic projections contact the same regions in these neurons as en passant connections [29]. Synaptic contact of partner neurons at inappropriate sites, as well as the formation of spatially isolated projection synapses, may deleteriously impact GFI information flow and circuit function [73,102].…”

“…The GF circuit connectivity can be mapped by injecting small tracers that pass through gap junctions to label the partner neurons [36]. We took advantage of this property by injecting the GFI axon with NB to test whether new, out-of-circuit neurons partner with supernumerary dfmr1 50M synaptic projections [29,37]. The w 1118 control and dfmr1 50M null dye coupling patterns, although complex, are extremely similar, with no newly labeled neurons appearing in the mutant condition ( Fig.…”

“…However, we cannot definitively state that GFC2 and 3 are the only synaptic targets of the excess projections. Moreover, it is interesting to note that GFC2 and 3 extensively contact the main axon shaft in both controls and dfmr1 nulls and form synapses there [29], making the projections unnecessary. This finding suggests either that the mutant condition drives the axon to seek out more synapses with its partners than it normally would require or that developmental projections normally pruned away during GF circuit maturation are inappropriately stabilized (Fig.…”

“…We focus particularly on the GF interneuron (GFI), a bilaterally symmetric neuron pair, with cell bodies and dendrites in the central brain, and large axons projecting into the thoracic ganglia [ 28 ]. The GFI axons use mixed electrical and chemical synapses to connect first with the peripherally synapsing interneuron (PSI) and giant fiber coupled 1–4 (GFC1–4) neurons at the inframedial bridge (IB), and then diverge to form two large bends that synapse onto the tergotrochanteral motor neuron (TTMn) and GFC2–3 neurons [ 27 , 29 – 31 ].…”