Neuromuscular junctions (NMJs) of third instar Act57B^E84K mutant larvae show aberrant active zone spacing and density. Homozygous Act57B^E84K larvae display reduced locomotion and growth, and rarely survive to pupation. Homozygous Act57B^E84K mutants are smaller than age-matched control animals, however, total synaptic bouton number per muscle are is not significantly different from control. Total active zone number normalised to bouton number reveals a reduction compared with controls. Bouton volume is slightly increased compared with controls, indicating that the reduced number of active zones per bouton in Act57B^E84K mutants is not due to a reduction in bouton volumes.

Act57B^E84K/Df(2R)Exel7166 larvae show a similar lethal stage and presynaptic active zone abnormalities.

Homozygous Act57B^E84K mutant larvae also exhibit changes relative to wild-type in the size and location of glutamate receptor fields.

Sarcomere and myofibril assembly is not disrupted in Act57B^E84K mutants. However, aberrant actin filaments are apparent outside of the sarcomeric organisation, concentrating around the NMJ. Instead of the normal homogeneous distribution of actin surrounding boutons in control animals, Act57B^E84K mutants display large actin swirls.

Act57B^E84K mutant larvae display defects in localisation of postsynaptic spectrin cytoskeleton components. The organised spectrin halo structure observed in wild-type animals is largely lost, leading to aberrant localisation of the spectrin cytoskeleton in Act57B^E84K mutants. These data indicate the actin-spectrin lattice at the muscle plasma membrane is disrupted in Act57B^E84K mutants.

Act57B^E84K mutant NMJs display a severe disruption of the postsynaptic subsynaptic reticulum (SSR) membrane folds that normally surround presynaptic boutons. Wild-type NMJs show numerous membrane folds surrounding each bouton, while Act57B^E84K boutons show a smaller SSR with fewer membrane layers and a reduction in thickness. Occasionally, membrane folds are missing entirely, and regions of presynaptic membrane are seen directly abutting muscle myofibrils in Act57B^E84K mutants.

Presynaptic active zones unapposed to postsynaptic glutamate receptor fields are observed in wild-type animals at a relatively low rate. In contrast, Act57B^E84K mutants display frequent defects in proper active zone alignment.

Compared with wild-type, Act57B^E84K/Df(2R)Exel7166 and homozygous Act57B^E84K mutants display significantly decreased evoked neurotransmitter release. Measurement of spontaneous miniature event amplitude reveals a reduction in the average size of the miniature excitatory junctional potential (mEJP or mini) compared with control indicating a postsynaptic defect. Consistent with the reduction in AZ number, mini frequency is also decreased in mutant animals. Quantal content(the number of synaptic vesicles released following an action potential) is similar in wild-type and mutant animals. Consistently, ultrastructural analysis fails to reveal any differences in synaptic vesicle size between wild-type and mutant animals.

Act57B^E84K, Nrx-1[+]/Nrx-1²⁷⁸ has abnormal neuroanatomy | third instar larval stage phenotype