Title: Characterization of an importin beta11 mutant at the Drosophila neuromuscular junction
Description: Cells adapt to changes in their environment often times through alterations in gene transcription. Factors that regulate gene transcription are actively transported into the nucleus through the nuclear pore complex to mediate these alterations. Nuclear import is therefore regulated by a class of proteins called importins, but these proteins have also received attention for their roles in synapse to nucleus signaling, cytoplasmic hold of signaling factors, and chaperone function in the cytoplasm. In a forward genetic screen for defective synaptic transmission, we isolated mutations in Drosophila importin β11, also called ran-binding protein 11. To date, no mutants of importin β11 have been characterized. In this dissertation, the effects of mutations in importin β11 in the fly nervous system are described. We found that homozygous mutants of importin β11 died at late pupal stages, which appeared to be caused by neuronal defects. We identified deficits in synaptic transmission of adult photoreceptors and at larval neuromuscular junctions. Photoreceptors were found to project to the appropriate targets in the mutant eye, however, the synaptic arbor on larval muscles had fewer boutons or synaptic varicosities. Synaptic markers displayed normal localization at the mutant neuromuscular junctions, but pMAD, a component of the signaling pathway of Bone Morphogenic Proteins (BMP) was decreased at the terminals of importin β11 mutants.

BMP signaling is critical for Drosophila development as mutations in BMP components lead to significant impairments in neuromuscular junction development and synaptic transmission. Although, pMAD is thought to function in the nucleus to regulate transcription, it is generated at and therefore localizes to the synapse. In importin β11 mutants, pMAD was detected at normal levels in mutant motor neuron nuclei, however, pMAD levels were markedly reduced at the synapse. Restoring pMAD to mutant motor neuron terminals rescued both synaptic strength and morphological deficits. Thus, importin β11 appears to modulate the development, maintenance, and function of the neuromuscular junction by regulating the concentration of pMAD at the neuromuscular synapse.