Subpial neuronal migration in the medulla oblongata of Pax-6-deficient rats

Downstream genes of Pax6 revealed by comprehensive transcriptome profiling in the developing rat hindbrain

BACKGROUND

The transcription factor Pax6 is essential for the development of the central nervous system and it exerts its multiple functions by regulating the expression of downstream target molecules. To screen for genes downstream of Pax6, we performed comprehensive transcriptome profiling analyses in the early hindbrain of Pax6 homozygous mutant and wild-type rats using microarrays.

RESULTS

Comparison of quadruplicate microarray experiments using two computational methods allowed us to identify differentially expressed genes that have relatively small fold changes or low expression levels. Gene ontology analyses of the differentially expressed molecules demonstrated that Pax6 is involved in various signal transduction pathways where it regulates the expression of many receptors, signaling molecules, transporters and transcription factors. The up- or down-regulation of these genes was further confirmed by quantitative RT-PCR. In situ staining of Fabp7, Dbx1, Unc5h1 and Cyp26b1 mRNAs showed that expression of these transcripts not only overlapped with that of Pax6 in the hindbrain of wild-type and Pax6 heterozygous mutants, but also was clearly reduced in the hindbrain of the Pax6 homozygous mutant. In addition, the Pax6 homozygous mutant hindbrain showed that Cyp26b1 expression was lacked in the dorsal and ventrolateral regions of rhombomeres 5 and 6, and that the size of rhombomere 5 expanded rostrocaudally.

CONCLUSIONS

These results indicate that Unc5h1 and Cyp26b1 are novel candidates for target genes transactivated by Pax6. Furthermore, our results suggest the interesting possibility that Pax6 regulates anterior-posterior patterning of the hindbrain via activation of Cyp26b1, an enzyme that metabolizes retinoic acid.

Expression study of cadherin7 and cadherin20 in the embryonic and adult rat central nervous system

BACKGROUND

Vertebrate classic cadherins are divided into type I and type II subtypes, which are individually expressed in brain subdivisions (e.g., prosomeres, rhombomeres, and progenitor domains) and in specific neuronal circuits in region-specific manners. We reported previously the expression of cadherin19 (cad19) in Schwann cell precursors. Cad19 is a type II classic cadherin closely clustered on a chromosome with cad7 and cad20. The expression patterns of cad7 and cad20 have been reported previously in chick embryo but not in the developing and adult central nervous system of mammals. In this study, we identified rat cad7 and cad20 and analyzed their expression patterns in embryonic and adult rat brains.

RESULTS

Rat cad7 protein showed 92% similarity to chick cad7, while rat cad20 protein had 76% similarity to Xenopus F-cadherin. Rat cad7 mRNA was initially expressed in the anterior neural plate including presumptive forebrain and midbrain regions, and then accumulated in cells of the dorsal neural tube and in rhombomere boundary cells of the hindbrain. Expression of rat cad20 mRNA was specifically localized in the anterior neural region and rhombomere 2 in the early neural plate, and later in longitudinally defined ventral cells of the hindbrain. The expression boundaries of cad7 and cad20 corresponded to those of region-specific transcription factors such as Six3, Irx3 and Otx2 in the neural plate, and Dbx2 and Gsh1 in the hindbrain. At later stages, the expression of cad7 and cad20 disappeared from neuroepithelial cells in the hindbrain, and was almost restricted to postmitotic cells, e.g. somatic motor neurons and precerebellar neurons. These results emphasized the diversity of cad7 and cad20 expression patterns in different vertebrate species, i.e. birds and rodents.

CONCLUSION

Taken together, our findings suggest that the expression of cad7 and cad20 demarcates the compartments, boundaries, progenitor domains, specific nuclei and specific neural circuits during mammalian brain development.

Hindbrain Rhombic Lip Is Comprised of Discrete Progenitor Cell Populations Allocated by Pax6

The lower rhombic lip (LRL) is a germinal zone in the dorsal hindbrain productive of tangentially migrating neurons, streaming extramurally (mossy fiber neurons) or intramurally (climbing fiber neurons). Here we show that LRL territory, operationally defined by Wnt1 expression, is parceled into molecular subdomains predictive of cell fate. Progressing dorsoventrally, Lmx1a and Gdf7 expression identifies the primordium for hindbrain choroid plexus epithelial cells; Math1, for mossy fiber neurons; and immediately ventral to Math1 yet within Wnt1(+) territory, a climbing fiber primordium dominated by Ngn1-expressing cells. Elimination of Pax6 results in expansion of this Ngn1(+) progenitor pool and reduction in the Math1(+) pool, with accompanying later enlargement of the climbing fiber nucleus and reductions in mossy fiber nuclei. Pax6 loss also disrupts Msx expression cell-nonautonomously, suggesting Pax6 may influence LRL progenitor identity indirectly through potentiating BMP signaling. These studies suggest that underlying the diversity and proportions of fates produced by the LRL is a precise suborganization regulated by Pax6.

The SDF-1/CXCR4 pathway and the development of the cerebellar system

Mice deficient for the chemokine receptor CXCR4 show premature translocation of granule cell neuroblasts from their germinal zone into the nascent cerebellum [Y.-R. Zuo et al. (1998) Nature, 393, 595-599]. Here, we used CXCR4-null mice to analyse the early development of cerebellar cortical inhibitory interneurons and pontine neurons which, in the adult, are synaptically integrated with granule cells. Cortical inhibitory interneuronal precursors normally invade the cerebellar anlage of CXCR4-deficient mice, but their dispersal is impeded by dislocated foci of proliferating granule cells, from which they are excluded. This is reminiscent of the strict exclusion of inhibitory interneuronal precursors from the superficial external granule cell layer. As inhibitory interneuronal precursors readily mingle with post-mitotic granule cells both in wild-type and CXCR4-null mice, these findings indicate that the developmentally regulated interactions between granule and inhibitory interneuronal precursors are independent of SDF-1/CXCR4 signalling. In contrast, the transit of pontine neurons from the rhombic lip through the anterior extramural stream to the basilar pons is disrupted in CXCR4-deficient animals. Migrating pontine neurons express CXCR4, and in CXCR4-null animals these cells are found displaced deep into the brainstem. Consequently, nascent pontine nuclei in CXCR4-deficient animals are hypoplastic. Moreover, they fail to express plexin D1, suggesting that SDF-1/CXCR4 signalling may also impinge on axon guidance critical to the orderly formation of granule cell mossy fibre afferents.

Deficits in motor coordination with aberrant cerebellar development in mice lacking testicular orphan nuclear receptor 4

Since testicular orphan nuclear receptor 4 (TR4) was cloned, its physiological function has remained largely unknown. Throughout postnatal development, TR4-knockout (TR4-/-) mice exhibited behavioral deficits in motor coordination, suggesting impaired cerebellar function. Histological examination of the postnatal TR4-/- cerebellum revealed gross abnormalities in foliation; specifically, lobule VII in the anterior vermis was missing. Further analyses demonstrated that the laminations of the TR4-/- cerebellar cortex were changed, including reductions in the thickness of the molecular layer and the internal granule layer, as well as delayed disappearance of the external granule cell layer (EGL). These lamination irregularities may result from interference with granule cell proliferation within the EGL, delayed inward migration of postmitotic granule cells, and a higher incidence of apoptotis. In addition, abnormal development of Purkinje cells was observed in the postnatal TR4-/- cerebellum, as evidenced by aberrant dendritic arborization and reduced calbindin staining intensity. Expression of Pax-6, Sonic Hedgehog (Shh), astrotactin (Astn), reelin, and Cdk-5, genes correlated with the morphological development of the cerebellum, is reduced in the developing TR4-/- cerebellum. Together, our findings suggest that TR4 is required for normal cerebellar development.

Lower rhombic lip-derived cells undergo transmedian tangential migration followed by radial migration in the chick embryo brainstem

Migration behaviour and fate of cells originated from the lower rhombic lip (LRL) was examined in the chick embryo hindbrain. LRL-derived cells tangentially migrate along the pial surface of the brainstem and form a transient subpial migratory stream. In the initial stages of migration, LRL-derived cells appose each other or axon-like processes, which is indicative of mode of homophilic chain migration and/or axophilic migration. Some LRL-derived cells relocate rostroventrally towards the pontine region, although the majority of them migrate circumferentially to the ventral medulla oblongata. Depending on the stage of generation, LRL-derived cells undergo transmedian migration; late-generated LRL-derived cells preferentially colonize the contralateral brainstem compared with early generated cells. Thus, latecomer neuron precursors may migrate past their predecessors in the migratory stream. When LRL-derived cells leave the subpial migratory stream, they change their migratory direction to a radial one and relocate inwardly, with a profile that resembles a tangential-to-radial change seen in cerebellar granule cell precursors. After they enter the parenchymal region of the brainstem, they exhibited morphological differentiation, and some differentiate into excitatory neurons. The present results suggest that LRL-derived cells migrate across boundaries such as midline or rhombomere, which may facilitate to build up cellular and functional architectures of the hindbrain.