Figure 7. Anatomical phenotypes of the giant fiber synaptic terminals in nrg mutants.

(A) GF synaptic terminals were visualized by injection of Rhodamine-dextran (red) into the GF. Dye-coupling of the GF to its target neurons, the Tergo Trochanteral motoneuron (TTMn), and the peripheral synapsing interneuron (PSI) via co-injection of Biotin (green) allows the detection of gap junctions between these neurons. In w¹¹¹⁸, nrg ¹⁴; P[nrg^wt] and nrg ¹⁴; P[nrg167^ΔFIGQY], a normal, large GF terminal was present and we observed dye-coupling with the TTMn and the PSI. In nrg ¹⁴; P[nrg180^Y-F], nrg ¹⁴; P[nrg180^Y-A], and nrg ¹⁴; P[nrg180^ΔFIGQY] mutants, the presynaptic terminal of the GF exhibited variable abnormal morphologies. They were thinner or swollen and contained large vacuole-like structures. However, in most cases the GF still dye-coupled with the postsynaptic target, the TTMn, and the PSI. Scale bar, 15 µm. (B) Quantification of morphological defects in w¹¹¹⁸ flies and nrg mutants. Only mutations affecting the Nrg180–FIGQY motif resulted in severe GF terminal aberrations. (C) Quantification of GF-to-TTMn dye-coupling (black bars) and comparison to animals with no electrophysiological responses (red bars) of the TTM with GF stimulation in the brain. A large percentage of animals expressing mutant versions of Nrg180–FIGQY proteins completely lacked electrophysiological responses despite the presence of dye-coupling, demonstrating a severe functional defect in these animals.