Rab81/Rab8B229 mutants show pharate lethality.
All the tested trans-heterozygous allelic combinations of Rab8B229 mutants display synaptic overgrowth, characterized by significant increase in synaptic bouton number of up to 100% relative to wild-type.
In addition to elevated bouton number, Rab81/Rab8B229 mutants also have a significant increase in neuromuscular junction length, branching, and satellite bouton number, with reduction in synaptic bouton size.
Postsynaptic expression of Rab8Scer\UAS.cWa under the control of Scer\GAL4Mhc.PW rescues the increased synaptic bouton number phenotype at muscle 6/7, but not synapse length at neuromuscular junctions in Rab81/Rab8B229 mutants.
Synaptic bouton number or length in muscle 4 is not rescued by muscle expression of Rab8Scer\UAS.cWa.
Rab81/Rab8B229 mutants display a clear reduction in both fast and slow recycling endosomes at both the NMJ and in motor neuron cell bodies in the larval ventral nerve cord. Upon ultrastructural analysis of mutant synapses, accumulation of large endosome-like, intermediary structures within synaptic boutons are observed. The observed structures, however, are dissimilar and unlikely synaptic vesicles as they are not released, because there is no increase in quantal size during electrophysiological analysis. On the contrary, Rab81/Rab8B229 mutants show decreased quantal size compared with wild-type. These structures are similar to autophagosomal intermediates and they are never observed in wild-type animals.