YFP expression in motor neurons subtly disrupts normal neuronal morphology and alters responses to dying-back pathology but not Wallerian degeneration.

<p>A–D – Representative confocal micrographs of NMJs in the levator auris longus muscle from a YFP-H mouse labelled for neurofilaments (NF: red) and postsynaptic motor endplates (BTX: blue). Note that all motor endplates were innervated, but only a small proportion of motor axons were YFP-positive in these mice (A). Panels B–D show high power micrographs of NMJs identified in panel A. Note abnormal accumulations of NF only in the motor nerve terminals of YFP-positive NMJs (B = NF accumulation score of 0, C = 3, D = 5). E – Bar chart (mean ± s.e.m.) showing quantification of NF accumulation in motor nerve terminals from YFP-H mice (0 = no accumulation; 5 = large abnormal accumulation), revealing a significant increase in NF accumulation in YFP-positive terminals (N = 5 mice, n = 9 muscles; *** P<0.001 Mann-Whitney test). F–G – Representative confocal micrographs of intramuscular axons supplying the transversus abdominis muscle from YFP-H mice (also labelled for NFs; red), before (F) and 20 hours after (G) intercostal nerve lesion. The presence of YFP did not alter the rate or morphological appearance of Wallerian degeneration after nerve injury, with NF fragmentation occurring in YFP-positive and –negative axons in all nerves examined (N = 6 mice, n = 6 nerves). H–K - Representative confocal micrographs of two NMJs in the levator auris longus muscle from a late-symptomatic (P24) <i>wasted</i>/YFP-H mouse labelled to reveal NFs (red) and postsynaptic motor endplates (BTX: blue). Note how the motor axon with YFP (bottom NMJ) remained intact whereas the motor axon without YFP (top) was undergoing retraction, characteristic of a dying-back pathology. L - Bar chart showing quantification of dying-back pathology at the NMJ in late-symptomatic (P24) <i>wasted</i>/YFP-H mice, revealing a significant attenuation of dying-back pathology in motor nerve terminals where YFP was present (i.e. a retention of endplates fully occupied by overlying NFs and a reduction in the numbers of partially occupied endplates; N = 4 mice, n = 7 muscles; *** P<0.001 Mann-Whitney test). Scale bars = 80 µm (A), 40 µm (B–D), 30 µm (F–G), 50 µm (H–K).