Morphogenetic and cellular movements that shape the mouse cerebellum; insights from genetic fate mapping.

We used the cerebellum as a model to study the morphogenetic and cellular processes underlying the formation of elaborate brain structures from a simple neural tube, using an inducible genetic fate mapping approach in mouse. We demonstrate how a 90 degrees rotation between embryonic days 9 and 12 converts the rostral-caudal axis of dorsal rhombomere 1 into the medial-lateral axis of the wing-like bilateral cerebellar primordium. With the appropriate use of promoters, we marked specific medial-lateral domains of the cerebellar primordium and derived a positional fate map of the murine cerebellum. We show that the adult medial cerebellum is produced by expansion, rather than fusion, of the thin medial primordium. Furthermore, ventricular-derived cells maintain their original medial-lateral coordinates into the adult, whereas rhombic lip-derived granule cells undergo lateral to medial posterior transverse migrations during foliation. Thus, we show that progressive changes in the axes of the cerebellum underlie its genesis.

Genetic subdivision of the tectum and cerebellum into functionally related regions based on differential sensitivity to engrailed proteins.

The genetic pathways that partition the developing nervous system into functional systems are largely unknown. The engrailed (En) homeobox transcription factors are candidate regulators of this process in the dorsal midbrain (tectum) and anterior hindbrain (cerebellum). En1 mutants lack most of the tectum and cerebellum and die at birth, whereas En2 mutants are viable with a smaller cerebellum and foliation defects. Our previous studies indicated that the difference in phenotypes is due to the earlier expression of En1 as compared with En2, rather than differences in protein function, since knock-in mice expressing En2 in place of En1 have a normal brain. Here, we uncovered a wider spectrum of functions for the En genes by generating a series of En mutant mice. First, using a conditional allele we demonstrate that En1 is required for cerebellum development only before embryonic day 9, but plays a sustained role in forming the tectum. Second, by removing the endogenous En2 gene in the background of En1 knock-in alleles, we show that Drosophila en is not sufficient to sustain midbrain and cerebellum development in the absence of En2, whereas En2 is more potent than En1 in cerebellum development. Third, based on a differential sensitivity to the dose of En1/2, our studies reveal a genetic subdivision of the tectum into its two functional systems and the medial cerebellum into four regions that have distinct circuitry and molecular coding. Our study suggests that an ;engrailed code' is integral to partitioning the tectum and cerebellum into functional domains.

Genetic and comparative mapping of genes dysregulated in mouse hearts lacking the Hand2 transcription factor gene.

The helix-loop-helix transcription factor HAND2 plays a vital role in the development of the heart, limb, facies, and other neural crest-derived structures. We used differential display analysis to identify 33 putative HAND2-regulated ESTs that are differentially expressed in Hand2(-/-) vs wild-type mice. We determined the positions on mouse and human genetic maps of 29 of these by using the T31 mouse Radiation Hybrid panel, comparison to human genomic sequence, and comparative mapping. We examined the conserved chromosomal locations for phenotypes that involve development of heart, face, and limb structures that are affected by HAND2. One EST mapped to a region of conserved synteny between mouse chromosome 2 and human chromosome 10p. RACE analysis extended the sequence and identified this cDNA as the mouse ortholog of human nebulette, an actin-binding protein expressed in fetal heart. Nebulette was shown to be deleted in DiGeorge Syndrome 2 patients with the proximal deletion of human 10p13-p14 that is associated with cardiac and craniofacial abnormalities.