Delphine Valloton

The corpus callosum (CC) is the major commissure that bridges the cerebral hemispheres. Agenesis of the CC is associated with human ciliopathies, but the origin of this default is unclear. Regulatory Factor X3 (RFX3) is a transcription factor involved in the control of ciliogenesis, and Rfx3-deficient mice show several hallmarks of ciliopathies including left-right asymmetry defects and hydrocephalus. Here we show that Rfx3-deficient mice suffer from CC agenesis associated with a marked disorganisation of guidepost neurons required for axon pathfinding across the midline. Using transplantation assays, we demonstrate that abnormalities of the mutant midline region are primarily responsible for the CC malformation. Conditional genetic inactivation shows that RFX3 is not required in guidepost cells for proper CC formation, but is required before E12.5 for proper patterning of the cortical septal boundary and hence accurate distribution of guidepost neurons at later stages. We observe focused but consistent ectopic expression of Fibroblast growth factor 8 (Fgf8) at the rostro commissural plate associated with a reduced ratio of GLIoma-associated oncogene family zinc finger 3 (GLI3) repressor to activator forms. We demonstrate on brain explant cultures that ectopic FGF8 reproduces the guidepost neuronal defects observed in Rfx3 mutants. This study unravels a crucial role of RFX3 during early brain development by indirectly regulating GLI3 activity, which leads to FGF8 upregulation and ultimately to disturbed distribution of guidepost neurons required for CC morphogenesis. Hence, the RFX3 mutant mouse model brings novel understandings of the mechanisms that underlie CC agenesis in ciliopathies.

BACKGROUND: Mammalian microRNAs (miRNAs) are sometimes subject to adenosine-to-inosine RNA editing, which can lead to dramatic changes in miRNA target specificity or expression levels. However, although a few miRNAs are known to be edited at identical positions in human and mouse, the evolution of miRNA editing has not been investigated in detail. In this study, we identify conserved miRNA editing events in a range of mammalian and non-mammalian species. RESULTS: We demonstrate deep conservation of several site-specific miRNA editing events, including two that date back to the common ancestor of mammals and bony fishes some 450 million years ago. We also find evidence of a recent expansion of an edited miRNA family in placental mammals and show that editing of these miRNAs is associated with changes in target mRNA expression during primate development and aging. While global patterns of miRNA editing tend to be conserved across species, we observe substantial variation in editing frequencies depending on tissue, age and disease state: editing is more frequent in neural tissues compared to heart, kidney and testis; in older compared to younger individuals; and in samples from healthy tissues compared to tumors, which together suggests that miRNA editing might be associated with a reduced rate of cell proliferation. CONCLUSIONS: Our results show that site-specific miRNA editing is an evolutionarily conserved mechanism, which increases the functional diversity of mammalian miRNA transcriptomes. Furthermore, we find that although miRNA editing is rare compared to editing of long RNAs, miRNAs are greatly overrepresented among conserved editing targets.

The NG2(+) glia, also known as polydendrocytes or oligodendrocyte precursor cells, represent a new entity among glial cell populations in the central nervous system. However, the complete repertoire of their roles is not yet identified. The embryonic NG2(+) glia originate from the Nkx2.1(+) progenitors of the ventral telencephalon. Our analysis unravels that, beginning from E12.5 until E16.5, the NG2(+) glia populate the entire dorsal telencephalon. Interestingly, their appearance temporally coincides with the establishment of blood vessel network in the embryonic brain. NG2(+) glia are closely apposed to developing cerebral vessels by being either positioned at the sprouting tip cells or tethered along the vessel walls. Absence of NG2(+) glia drastically affects the vascular development leading to severe reduction of ramifications and connections by E18.5. By revealing a novel and fundamental role for NG2(+) glia, our study brings new perspectives to mechanisms underlying proper vessels network formation in embryonic brains.

Abstract The homeodomain transcription factor Nkx2.1 (NK2 homeobox 1) controls cell differentiation of telencephalic GABAergic interneurons and oligodendrocytes. Here we show that Nkx2.1 also regulates astrogliogenesis of the telencephalon from embryonic day (E) 14.5 to E16.5. Moreover we identify the different mechanisms by which Nkx2.1 controls the telencephalic astrogliogenesis. In Nkx2.1 knockout ( Nkx2.1 −/− ) mice a drastic loss of astrocytes is observed that is not related to cell death. Further, in vivo analysis using BrdU incorporation reveals that Nkx2.1 affects the proliferation of the ventral neural stem cells that generate early astrocytes. Also, in vitro neurosphere assays showed reduced generation of astroglia upon loss of Nkx2.1, which could be due to decreased precursor proliferation and possibly defects in glial specification/differentiation. Chromatin immunoprecipitation analysis and in vitro co-transfection studies with an Nkx2.1-expressing plasmid indicate that Nkx2.1 binds to the promoter of glial fibrillary acidic protein (GFAP), primarily expressed in astrocytes, to regulate its expression. Hence, Nkx2.1 controls astroglial production spatiotemporally in embryos by regulating proliferation of the contributing Nkx2.1-positive precursors.