ABSTRACT
Fragile X mental retardation gene (FMR1) encodes an RNA binding protein that acts as a negative translational regulator. We have developed a Drosophila fragile X syndrome model using loss-of-function mutants and overexpression of the FMR1 homolog (dfxr). dfxr nulls display enlarged synaptic terminals, whereas neuronal overexpression results in fewer and larger synaptic boutons. Synaptic structural defects are accompanied by altered neurotransmission, with synapse type-specific regulation in central and peripheral synapses. These phenotypes mimic those observed in mutants of microtubule-associated Futsch. Immunoprecipitation of dFXR shows association with futsch mRNA, and Western analyses demonstrate that dFXR inversely regulates Futsch expression. dfxr futsch double mutants restore normal synaptic structure and function. We propose that dFXR acts as a translational repressor of Futsch to regulate microtubule-dependent synaptic growth and function.
Subject(s)
Drosophila melanogaster/physiology , Microtubule-Associated Proteins/metabolism , Nerve Growth Factors/metabolism , Nerve Tissue Proteins/metabolism , Neuromuscular Junction/physiology , RNA-Binding Proteins , Synapses/physiology , Animals , Brain/cytology , Brain/physiology , Drosophila Proteins/genetics , Drosophila Proteins/metabolism , Drosophila melanogaster/genetics , Electroretinography , Evoked Potentials/physiology , Flight, Animal/physiology , Fragile X Mental Retardation Protein , Gene Expression Regulation , Genes, Insect , Humans , Microscopy, Fluorescence , Microtubule-Associated Proteins/genetics , Molecular Sequence Data , Nerve Growth Factors/genetics , Nerve Tissue Proteins/genetics , Neuromuscular Junction/cytology , Neurons/cytology , Neurons/physiology , Patch-Clamp Techniques , Photoreceptor Cells, Invertebrate/growth & development , Photoreceptor Cells, Invertebrate/physiologyABSTRACT
We describe here the cloning and functional characterization of a neural-specific novel member of the Ig superfamily, turtle (tutl), with a structure of five Ig C2-type domains, two fibronectin type III domains, and one transmembrane region. Alternative splicing of the tutl gene produces at least four Tutl isoforms, including two transmembrane proteins and two secreted proteins, with primary structures closely related to a human brain protein (KIAA1355), the Deleted in Colorectal Cancer/Neogenin/Frazzled receptor family, and the Roundabout/Dutt1 receptor family. An allelic series of tutl gene mutations resulted in recessive lethality to semilethality, indicating that the gene is essential. In contrast to other family members, tutl does not play a detectable role in axon pathfinding or nervous system morphogenesis. Likewise, basal synaptic transmission and locomotory movement are unaffected. However, tutl mutations cause striking movement defects exhibited in specific types of highly coordinated behavior. Specifically, tutl mutants display an abnormal response to tactile stimulation, the inability to regain an upright position from an inverted position (hence, "turtle"), and the inability to fly in adulthood. These phenotypes demonstrate that tutl plays an essential role in establishing a nervous system capable of executing coordinated motor output in complex behaviors.
