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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.

Septate and tight junctions are thought to seal neighboring cells together and to function as barriers between epithelial cells. We have characterized a novel member of the neurexin family, Neurexin IV (NRX), which is localized to septate junctions (SJs) of epithelial and glial cells. NRX is a transmembrane protein with a cytoplasmic domain homologous to glycophorin C, a protein required for anchoring protein 4.1 in the red blood cell. Absence of NRX results in mislocalization of Coracle, a Drosophila protein 4.1 homolog, at SJs and causes dorsal closure defects similar to those observed in coracle mutants. nrx mutant embryos are paralyzed, and electrophysiological studies indicate that the lack of NRX in glial-glial SJs causes a breakdown of the blood-brain barrier. Electron microscopy demonstrates that nrx mutants lack the ladder-like intercellular septa characteristic of pleated SJs (pSJs). These studies identify NRX as the first transmembrane protein of SJ and demonstrate a requirement for NRX in the formation of septate-junction septa and intercellular barriers.

Motor function requires that motor axons extend from the spinal cord at regular intervals and that they are myelinated by Schwann cells. Little attention has been given to another cellular structure, the perineurium, which ensheaths the motor nerve, forming a flexible, protective barrier. Consequently, the origin of perineurial cells and their roles in motor nerve formation are poorly understood. Using time-lapse imaging in zebrafish, we show that perineurial cells are born in the CNS, arising as ventral spinal-cord glia before migrating into the periphery. In embryos lacking perineurial glia, motor neurons inappropriately migrated outside of the spinal cord and had aberrant axonal projections, indicating that perineurial glia carry out barrier and guidance functions at motor axon exit points. Additionally, reciprocal signaling between perineurial glia and Schwann cells was necessary for motor nerve ensheathment by both cell types. These insights reveal a new class of CNSborn glia that critically contributes to motor nerve development.The formation of spinal motor nerves requires coordinated interactions between several types of cells. Motor neurons extend axons into developing muscle fields from the spinal cord through segmentally positioned motor exit points (MEPs). These axons encounter neural crest-derived boundary cap cells clustered at MEPs, which permit the axons, but not the cell bodies, to exit from the spinal cord 1 . As motor axons approach their targets, they are sequentially wrapped and then myelinated by Schwann cells, glial cells that also develop from neural crest.The myelinated motor nerve is surrounded by a flexible cellular sheath called the perineurium, first described in 1841 by Henle and later named by Key and Retzius 2 . The perineurium consists of uninterrupted, concentric rings of flattened cells that are connected by tight junctions and encase motor nerves from the MEP to the neuromuscular junction (NMJ) 2-7 . The perineurial sheath serves as a barrier, protecting axons from ionic flux, toxins and infection 4, 8-10 . Therefore, formation of the perineurium is essential for peripheral nerve function.Correspondence should be addressed to B.A. (b.appel@vanderbilt.edu). AUTHOR CONTRIBUTIONS S.K. produced all of the data except for the electron microscopy images shown in Figure 5c NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptPrevious studies have suggested that, during development, the perineurium appears to form by a series of steps in which nearby mesenchymal cells first assemble as a loosely organized tube around the nerve and then mature to create a multilayered barrier 11 . The maturation step requires the signaling molecule Desert hedgehog (Dhh), which is expressed by Schwann cells. In the absence of Dhh, the perineurium is disorganized and is permeable to macromolecules and inflammatory cells 12 . Although these studies revealed a critical feature of perineurial cell differentiation, the origin of perineurial cells and how they initially associate ...

Fragile X Syndrome (FraX) is a broad-spectrum neurological disorder with symptoms ranging from hyperexcitability to mental retardation and autism. Loss of the fragile X mental retardation 1 (fmr1) gene product, the mRNA-binding translational regulator FMRP, causes structural over-elaboration of dendritic and axonal processes, as well as functional alterations in synaptic plasticity at maturity. It is unclear, however, whether FraX is primarily a disease of development, a disease of plasticity or both: a distinction that is vital for engineering intervention strategies. To address this crucial issue, we have used the Drosophila FraX model to investigate the developmental function of Drosophila FMRP (dFMRP). dFMRP expression and regulation of chickadee/profilin coincides with a transient window of late brain development. During this time, dFMRP is positively regulated by sensory input activity, and is required to limit axon growth and for efficient activity-dependent pruning of axon branches in the Mushroom Body learning/memory center. These results demonstrate that dFMRP has a primary role in activity-dependent neural circuit refinement during late brain development.