A forward genetic screen with a thalamocortical axon reporter mouse yields novel neurodevelopment mutants and a distinct emx2 mutant phenotype

Neural Dev. 2011 Jan 7;6:3. doi: 10.1186/1749-8104-6-3.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alkylating Agents / pharmacology
  • Animals
  • Axons / physiology*
  • Body Patterning / drug effects
  • Body Patterning / genetics
  • Cell Adhesion Molecules, Neuronal / genetics
  • Cell Adhesion Molecules, Neuronal / metabolism
  • Cerebral Cortex* / embryology
  • Cerebral Cortex* / growth & development
  • Cerebral Cortex* / metabolism
  • Embryo, Mammalian
  • Ethylnitrosourea / pharmacology
  • Extracellular Matrix Proteins / genetics
  • Extracellular Matrix Proteins / metabolism
  • Female
  • Gene Expression Regulation, Developmental / drug effects
  • Gene Expression Regulation, Developmental / genetics*
  • Genetic Testing / methods
  • Homeodomain Proteins / genetics*
  • Lac Operon / genetics
  • Male
  • Mice
  • Mice, Inbred BALB C
  • Mice, Inbred C57BL
  • Mice, Transgenic
  • Mutation / genetics*
  • Nerve Tissue Proteins / genetics
  • Nerve Tissue Proteins / metabolism
  • Neural Pathways / abnormalities
  • Neural Pathways / embryology
  • Neural Pathways / growth & development
  • Phenotype*
  • Serine Endopeptidases / genetics
  • Serine Endopeptidases / metabolism
  • Thalamus* / embryology
  • Thalamus* / growth & development
  • Thalamus* / metabolism
  • Transcription Factors / genetics*
  • beta-Galactosidase / metabolism

Substances

  • Alkylating Agents
  • Cell Adhesion Molecules, Neuronal
  • Extracellular Matrix Proteins
  • Homeodomain Proteins
  • Nerve Tissue Proteins
  • Transcription Factors
  • empty spiracles homeobox proteins
  • beta-Galactosidase
  • Serine Endopeptidases
  • reelin protein
  • Ethylnitrosourea