A mosaic genetic screen for genes necessary forDrosophilamushroom body neuronal morphogenesis

Abstract

Neurons undergo extensive morphogenesis during development. To systematically identify genes important for different aspects of neuronal morphogenesis, we performed a genetic screen using the MARCM system in the mushroom body (MB) neurons of the Drosophila brain. Mutations on the right arm of chromosome 2 (which contains ~20% of the Drosophila genome) were made homozygous in a small subset of uniquely labeled MB neurons. Independently mutagenized chromosomes (4600) were screened, yielding defects in neuroblast proliferation, cell size, membrane trafficking, and axon and dendrite morphogenesis. We report mutations that affect these different aspects of morphogenesis and phenotypically characterize a subset. We found that roadblock, which encodes a dynein light chain, exhibits reduced cell number in neuroblast clones, reduced dendritic complexity and defective axonal transport. These phenotypes are nearly identical to mutations in dynein heavy chain Dhc64 and in Lis1, the Drosophila homolog of human lissencephaly 1, reinforcing the role of the dynein complex in cell proliferation, dendritic morphogenesis and axonal transport. Phenotypic analysis of short stop/kakapo, which encodes a large cytoskeletal linker protein, reveals a novel function in regulating microtubule polarity in neurons. MB neurons mutant for flamingo, which encodes a seven transmembrane cadherin, extend processes beyond their wild-type dendritic territories. Overexpression of Flamingo results in axon retraction. Our results suggest that most genes involved in neuronal morphogenesis play multiple roles in different aspects of neural development, rather than performing a dedicated function limited to a specific process.

The atypical cadherin flamingo regulates synaptogenesis and helps prevent axonal and synaptic degeneration in Drosophila

“…Alternatively, these ectopic synapses might result from a defect in either synaptic targeting or nerve sprouting. Consistent with this idea, we note the ectopic synapse is reminiscent of aberrant axonal projections in the visual system Senti et al, 2003) and the mushroom body in the fmi mutant (Reuter et al, 2003) and in the brain and spinal cord of Celsr3-deficient mice (Tissir et al, 2005). Hence, Flamingo likely also regulates both synaptic target selection and axonal projection at the NMJ.…”

“…Defects in axonal projections in fmi mutants have been implicated at the embryonic NMJ and shown in sensory cells (Sweeney et al, 2002), in the fly visual system Senti et al, 2003), and mushroom bodies (Reuter et al, 2003). Additionally, mice deficient in Celsr3 have also been shown to have aberrant and overgrown axons in the brain and spinal cord (Tissir et al, 2005).…”

“…Flamingo was initially found in Drosophila to play a key role in planar cell polarity through the Frizzled-dependent signaling pathway Chae et al, 1999;Lu et al, 1999). In the Drosophila nervous system, it acts through a yet unidentified signaling pathway to regulate dendritic growth (Gao et al, 2000;Sweeney et al, 2002;Reuter et al, 2003), axonal projection, and target selection (Sweeney et al, 2002;Reuter et al, 2003;Lee et al, 2003;and Senti et al, 2003). Many of these functions are conserved in mammalian orthologs of Flamingo, Celsr (Curtin et al, 2003;Shima et al, 2004;Tissir et al, 2005).…”

Planar cell polarity genes, Celsr1-3, in neural development

“…Further work showed that Flamingo acts probably through two different mechanisms to regulate dendritic field formation [23] . At the initial stage, Using a mosaic genetic screen, flamingo has been found to regulate axonal projection in addition to suppressing dendritic overextension [24] . Flamingo overexpression in the mushroom body results in loss of the dorsal branches of axons.…”

Functional and Genetic Analysis of Spectraplakins in Drosophila

“…In this way, only the mutant neurons are labeled and their morphology can be analyzed. Notably, the shot 3 mutant allele was the first ever mutant allele used for MARCM analyses, which have revealed Shot requirements during axon growth and neuronal polarity at pupal stages (Lee & Luo, 1999;Reuter, 2003. …”