Plasticity of motor nerve terminals in Drosophila T (X,Y)V7 mutant: effect of deregulation of the novel calcium-binding protein frequenin.

The Drosophila T(X,Y)V7 mutant is characterized by abnormally large motor responses that build up upon repetitive stimulation. Genetically it is characterized by a chromosomal breakpoint located at the proximal end of the Shaker gene complex. This mutation affects a gene which encodes a novel calcium-binding protein: the frequenin. Since neuronal activity is known to affect neurite elongation we looked for the geometry of motor terminal arborization in this mutant. Our results show a significant reduction in number and length of motor terminal branches in mutants as compared to wild type. This observation is opposite to the effect of other hyperexcitable mutations such as Shaker or ether-a-gogo or Hyperkinetic. Thus the V7 phenotype cannot be interpreted as a result of changes in motoneuron firing pattern. According to results obtained on transformed larvae in which frequenin cDNA expression was under the control of a heat shock promoter, it appears that the morphological phenotype of V7 may be due to specific effects of deregulation of this calcium-binding protein.