ABSTRACT
In this study, we elucidate the roles of the winged-helix transcription factor Foxa2 in ventral CNS development in zebrafish. Through cloning of monorail (mol), which we find encodes the transcription factor Foxa2, and phenotypic analysis of mol-/- embryos, we show that floorplate is induced in the absence of Foxa2 function but fails to further differentiate. In mol-/- mutants, expression of Foxa and Hh family genes is not maintained in floorplate cells and lateral expansion of the floorplate fails to occur. Our results suggest that this is due to defects both in the regulation of Hh activity in medial floorplate cells as well as cell-autonomous requirements for Foxa2 in the prospective laterally positioned floorplate cells themselves. Foxa2 is also required for induction and/or patterning of several distinct cell types in the ventral CNS. Serotonergic neurones of the raphenucleus and the trochlear motor nucleus are absent in mol-/- embryos, and oculomotor and facial motoneurones ectopically occupy ventral CNS midline positions in the midbrain and hindbrain. There is also a severe reduction of prospective oligodendrocytes in the midbrain and hindbrain. Finally, in the absence of Foxa2, at least two likely Hh pathway target genes are ectopically expressed in more dorsal regions of the midbrain and hindbrain ventricular neuroepithelium, raising the possibility that Foxa2 activity may normally be required to limit the range of action of secreted Hh proteins.
Subject(s)
Central Nervous System/embryology , Embryonic Induction/physiology , Motor Neurons/cytology , Oligodendroglia/cytology , Transcription Factors/metabolism , Zebrafish Proteins/metabolism , Animals , Central Nervous System/cytology , Central Nervous System/physiology , Embryo, Nonmammalian/cytology , Embryo, Nonmammalian/metabolism , Forkhead Transcription Factors , Gene Expression Regulation, Developmental/genetics , Gene Expression Regulation, Developmental/physiology , Hedgehog Proteins , Motor Neurons/metabolism , Mutation/genetics , Oligodendroglia/metabolism , Raphe Nuclei/cytology , Raphe Nuclei/embryology , Raphe Nuclei/metabolism , Serotonin/metabolism , Trans-Activators/genetics , Trans-Activators/metabolism , Transcription Factors/genetics , Trochlear Nerve/cytology , Trochlear Nerve/embryology , Trochlear Nerve/metabolism , Zebrafish/embryology , Zebrafish/genetics , Zebrafish/metabolism , Zebrafish Proteins/geneticsABSTRACT
Hyperekplexia is a hereditary neurological disorder characterized by excessive startle responses. Within the disorder two clinical forms can be distinguished. The major form is characterized by continuous generalized stiffness in the first year of life and an exaggerated startle reflex, accompanied by temporary generalized stiffness and falls, whereas in the minor form only excessive startle and hypnic jerks have been described. Mutations in the gene encoding the alpha-1 subunit of the glycine receptor (GLRA1) are responsible for the major form of hyperekplexia but no mutation was detected in patients with the minor form in the large Dutch pedigree originally described by Suhren and colleagues. Here we describe the genetic analysis of the GLRA1 gene of two English families in which both forms of hyperekplexia were present. Mutation analysis revealed no genetic defect in the GLRA1 gene in patients carrying either the minor or major forms. This is further evidence that the minor form of hyperekplexia is seldom due to a genetic defect in the GLRA1 gene.
