title: A forward genetic screen with a thalamocortical axon reporter mouse yields novel neurodevelopment mutants and a distinct emx2 mutant phenotype.
abstract: BACKGROUND
  The dorsal thalamus acts as a gateway and modulator for information going to and from the cerebral cortex. This activity requires the formation of reciprocal topographic axon connections between thalamus and cortex. The axons grow along a complex multistep pathway, making sharp turns, crossing expression boundaries, and encountering intermediate targets. However, the cellular and molecular components mediating these steps remain poorly understood.
  
  
  RESULTS
  To further elucidate the development of the thalamocortical system, we first created a thalamocortical axon reporter line to use as a genetic tool for sensitive analysis of mutant mouse phenotypes. The TCA-tau-lacZ reporter mouse shows specific, robust, and reproducible labeling of thalamocortical axons (TCAs), but not the overlapping corticothalamic axons, during development. Moreover, it readily reveals TCA pathfinding abnormalities in known cortical mutants such as reeler. Next, we performed an unbiased screen for genes involved in thalamocortical development using random mutagenesis with the TCA reporter. Six independent mutant lines show aberrant TCA phenotypes at different steps of the pathway. These include ventral misrouting, overfasciculation, stalling at the corticostriatal boundary, and invasion of ectopic cortical cell clusters. An outcross breeding strategy coupled with a genomic panel of single nucleotide polymorphisms facilitated genetic mapping with small numbers of mutant mice. We mapped a ventral misrouting mutant to the Emx2 gene, and discovered that some TCAs extend to the olfactory bulbs in this mutant. Mapping data suggest that other lines carry mutations in genes not previously known for roles in thalamocortical development.
  
  
  CONCLUSIONS
  These data demonstrate the feasibility of a forward genetic approach to understanding mammalian brain morphogenesis and wiring. A robust axonal reporter enabled sensitive analysis of a specific axon tract inside the mouse brain, identifying mutant phenotypes at multiple steps of the pathway, and revealing a new aspect of the Emx2 mutant. The phenotypes highlight vulnerable choice points and latent tendencies of TCAs, and will lead to a refined understanding of the elements and interactions required to form the thalamocortical system.
authors.affiliation: Howard Hughes Medical Institute, Department of Neurology, Beth Israel Deaconess Medical Center, Boston, MA, USA. ndwyer@virginia.edu
authors.forename: Noelle D
authors.initials: ND
authors.lastname: Dwyer
authors.forename: Danielle K
authors.initials: DK
authors.lastname: Manning
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Moran
authors.forename: Raksha
authors.initials: R
authors.lastname: Mudbhary
authors.forename: Michael S
authors.initials: MS
authors.lastname: Fleming
authors.forename: Carlita B
authors.initials: CB
authors.lastname: Favero
authors.forename: Vita M
authors.initials: VM
authors.lastname: Vock
authors.forename: Dennis D M
authors.initials: DD
authors.lastname: O'Leary
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Walsh
authors.forename: David R
authors.initials: DR
authors.lastname: Beier
mesh_terms: D000477:Alkylating Agents / Q000494:pharmacology; D000818:Animals; D001369:Axons / Q000502:physiology*; D019521:Body Patterning / Q000187:drug effects / Q000235:genetics; D015816:Cell Adhesion Molecules, Neuronal / Q000235:genetics / Q000378:metabolism; D002540:Cerebral Cortex / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D004622:Embryo, Mammalian; D005038:Ethylnitrosourea / Q000494:pharmacology; D016326:Extracellular Matrix Proteins / Q000235:genetics / Q000378:metabolism; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects / Q000235:genetics*; D005820:Genetic Testing / Q000379:methods; D018398:Homeodomain Proteins / Q000235:genetics*; D007763:Lac Operon / Q000235:genetics; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009154:Mutation / Q000235:genetics*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism; D009434:Neural Pathways / Q000002:abnormalities / Q000196:embryology / Q000254:growth & development; D010641:Phenotype; D000091042:Reelin Protein; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism; D013788:Thalamus / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics*; D001616:beta-Galactosidase / Q000378:metabolism; D000099260:Empty Spiracles Homeobox Proteins
chemical_list: D000477:Alkylating Agents; D015816:Cell Adhesion Molecules, Neuronal; D005038:Ethylnitrosourea; D016326:Extracellular Matrix Proteins; D018398:Homeodomain Proteins; D009419:Nerve Tissue Proteins; D000091042:Reelin Protein; D012697:Serine Endopeptidases; D014157:Transcription Factors; D001616:beta-Galactosidase; D000099260:Empty Spiracles Homeobox Proteins