Development
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sox10 is necessary for development of neural and pigment cell derivatives of the neural crest (NC). However, whereas a direct role for Sox10 activity has been established in pigment and glial lineages, this is more controversial in NC-derived sensory neurons of the dorsal root ganglia (DRGs). We proposed that sox10 functioned in specification of sensory neurons, whereas others suggested that sensory neuronal defects were merely secondary to absence of glia. ⋯ Critically, we demonstrate that Sox10 is expressed transiently in the sensory neuron lineage, and specifies sensory neuron precursors by regulating the proneural gene neurogenin1. Consistent with this, we have isolated a novel sox10 mutant that lacks glia and yet displays a neurogenic DRG phenotype. In conjunction with previous findings, these data establish the generality of our model of Sox10 function in NC fate specification.
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The significant morbidity and mortality associated with laterality disease almost always are attributed to complex congenital heart defects (CHDs), reflecting the extreme susceptibility of the developing heart to disturbances in the left-right (LR) body plan. To determine how LR positional information becomes ;translated' into anatomical asymmetry, left versus right side cardiomyocyte cell lineages were traced in normal and laterality defective embryos of the frog, Xenopus laevis. In normal embryos, myocytes in some regions of the heart were derived consistently from a unilateral lineage, whereas other regions were derived consistently from both left and right side lineages. ⋯ Surprisingly, most of the embryos that failed to develop heterotaxy or situs inversus in response to misregulated ALK4 signaling nevertheless had altered Pitx2c expression, abnormal cardiomyocyte LR lineage composition and abnormal heart structure, demonstrating that cardiac laterality defects can occur even in instances of otherwise normal body situs. These results indicate that: (1) different regions of the heart contain distinct LR myocyte compositions; (2) LR cardiomyocyte lineages and Pitx2c expression are altered in laterality defective embryos; and (3) abnormal LR cardiac lineage composition frequently is associated with cardiac malformations. We propose that proper LR cell composition is necessary for normal morphogenesis, and that misallocated LR cell lineages may be causatively linked with CHDs that are present in heterotaxic individuals, as well as some 'isolated' CHDs that are found in individuals lacking overt features of laterality disease.
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Nuclei are aggregates of neurons distributed in the central nervous system and are fundamental functional units that share anatomical and physiological features. Despite their importance, the cellular basis that leads to nucleogenesis is only poorly understood. Using exo utero electroporation with an enhanced yellow fluorescent protein (EYFP) gene, we show that the precerebellar neurons derived from the lower rhombic lip (lRL) undergo multiple migration steps to form nuclei. ⋯ Before forming the clusters, EYFP-labelled neurons begin to migrate radially towards the ventricle in close apposition to nestin-positive radial fibres, and then they aggregate as they detach from the fibres. Inhibition of cadherin function in ECN and LRN progenitors caused ipsilateral formation of the ECN and LRN, implying that the transition of their migration from tangential to radial involves a cell-intrinsic mechanism. These observations suggest that nucleogenesis of precerebellar neurons is a result of multi-phasic migration, and that ventricle-directed radial glia-guided migration is a key step for nucleogenesis.
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Three major axon pathways cross the midline of the vertebrate forebrain early in embryonic development: the postoptic commissure (POC), the anterior commissure (AC) and the optic nerve. We show that a small population of Gfap+ astroglia spans the midline of the zebrafish forebrain in the position of, and prior to, commissural and retinal axon crossing. These glial ;bridges' form in regions devoid of the guidance molecules slit2 and slit3, although a subset of these glial cells express slit1a. ⋯ By contrast, reducing Slit1a function led to reduced midline axon crossing, suggesting a distinct role for Slit1a in midline axon guidance. Blocking Slit2 and Slit3, but not Slit1a, function in the Hh pathway mutant yot (gli2DR) dramatically rescued POC axon crossing and glial bridge formation at the midline, indicating that expanded Slit2 and Slit3 repellent function is largely responsible for the lack of midline crossing in these mutants. This analysis shows that Hh signaling helps to pattern the expression of Slit guidance molecules that then help to regulate glial cell position and axon guidance across the midline of the forebrain.
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Chromatin remodeling complexes play crucial roles in transcription and are implicated in processes including cell proliferation, differentiation and embryonic patterning. Brg1 is the catalytic subunit of the SWI/SNF chromatin remodeling complex and shows neural-enriched expression. Although early lethality of Brg1-null mice reflects its importance in embryogenesis, this phenotype precluded further study of specific Brg1-dependent developmental processes. ⋯ Consistent with this, dominant-negative interference with Brg1 function in P19 cells suppressed neuronal differentiation promoted by NeuroD2, showing the requirement of Brg1 for neuronal differentiation is conserved in mammalian cells. Finally, we discovered that Brg1 physically associates with both Ngnr1 and NeuroD and that interference with Brg1 function blocks Neurogenin3- and NeuroD2-mediated reporter gene transactivation. Together, our results demonstrate that Brg1 (and by inference the SWI/SNF complex) is required for neuronal differentiation by mediating the transcriptional activities of proneural bHLH proteins.