atl2 heterozygous larval ddaE neurons exhibit defects in axon regeneration, shown by a significant decrease in axon length at 96h after laser-induced severance, as compared to controls.
Endoplasmic reticulum (ER) crossbridges are almost completely eliminated and the long straight ER tubules (seen in wild type) become wavy in motor neuron initial axon segments of atl2/atl2 third instar larvae; in mutant motor neuron nerve terminals there is ER fragmentation (basket structures are eliminated and the ER signal is diffuse); there is little effect on ER morphology in mutant motor neuron cell bodies.
Excitatory junction potential (EJP) amplitude is significantly decreased (at 0.1 or 0.6 mM Ca[2+]) at the neuromuscular junction of atl2/atl2 third instar larvae compared to controls. As bath Ca[2+] concentration increases, transmitter release in atl2/atl2 larvae becomes progressively more similar to wild type (partial rescue at 1.5 mM). There is a small but significant increase in mini EJP (mEJP) amplitude in atl2/atl2 mutants (mean of 4 pooled Ca[2+] concentrations). Reduction in EJP corresponds with a decrease in the frequency of successful synaptic transmission in atl2/atl2 third instar larvae.
The number and size of active zones is not significantly different to wild type in atl2/atl2 mutants.
Homozygotes and atl2/Df(3R)Exel7357 animals survive only to pupal stages with few adult escapers (8%). The escapers have a smaller body size than controls and are sterile.
atl2/Df(3R)Exel7357 larvae show a mild defect in synaptic growth at the neuromuscular junction (NMJ); type Ib boutons are more clustered at the terminal regions of NMJ branches than in wild type, due to increased satellite bouton formation. Average bouton number per NMJ (at muscles 6/7 of abdominal segment 2) in atl2/Df(3R)Exel7357 larvae is approximately 17% higher at 25[o]C and approximately 42% higher at 18[o]C compared to controls. Although there is an increased in the total number of boutons, the proportion of Ib and Is boutons is unchanged compared to wild type.
The combined surface area of muscles 6 and 7 of segment A2 is reduced by approximately 31% at 25[o]C and is reduced by approximately 17% at 18[o]C in atl2/Df(3R)Exel7357 larvae compared to controls.
Bouton number normalised to muscle surface area is increased by approximately 70% in atl2/Df(3R)Exel7357 larvae at both 18 and 25[o]C.
The cross-sectional midline area of type Ib boutons at the NMJ of muscles 6/7 is decreased by approximately 44% compared to wild type in atl2/Df(3R)Exel7357 larvae.
atl2/Df(3R)Exel7357 neuromuscular junctions often form small satellite boutons. These satellite boutons share a common SSR with their parental boutons and contain clear synaptic vesicles and T-bars at active zones, suggesting that they may form functional synapses.
The subsynaptic reticulum (SSR) is less extensive than normal in atl2/Df(3R)Exel7357 larvae and is structurally disorganised. The boundary between the SSR and muscle cell-cytoplasm is not clearly defined. SSR thickness (the distance between the presynaptic membrane and the outermost boundary of the SSR) is reduced by 36%. There is a 53% reduction in the cross-sectional area of the SSR and a 66% reduction in the cross-sectional area of the SSR cisterna, resulting in a 27% decrease in the ratio of the SSR cisterna area to the SSR area compared to wild type. The density of the SSR membranes (number of membrane layers per μm) is unaltered.
At 25[o]C, homozygous larvae show an increase in bouton number/muscle area at the NMJ.
The body wall muscles of atl2/Df(3R)Exel7357 larvae contain more prominent microtubules than wild-type muscles.