Novel lines of Pax6-/- embryonic stem cells exhibit reduced neurogenic capacity without loss of viability

Pax6 mutant cerebral organoids partially recapitulate phenotypes of Pax6 mutant mouse strains

Cerebral organoids show great promise as tools to unravel the complex mechanisms by which the mammalian brain develops during embryogenesis. We generated mouse cerebral organoids harbouring constitutive or conditional mutations in Pax6, which encodes a transcription factor with multiple important roles in brain development. By comparing the phenotypes of mutant organoids with the well-described phenotypes of Pax6 mutant mouse embryos, we evaluated the extent to which cerebral organoids reproduce phenotypes previously described in vivo. Organoids lacking Pax6 showed multiple phenotypes associated with its activity in mice, including precocious neural differentiation, altered cell cycle and an increase in abventricular mitoses. Neural progenitors in both Pax6 mutant and wild type control organoids cycled more slowly than their in vivo counterparts, but nonetheless we were able to identify clear changes to cell cycle attributable to the absence of Pax6. Our findings support the value of cerebral organoids as tools to explore mechanisms of brain development, complementing the use of mouse models.

Nuclear Transporter IPO13 Is Central to Efficient Neuronal Differentiation

Molecular transport between the nucleus and cytoplasm of the cell is mediated by the importin superfamily of transport receptors, of which the bidirectional transporter Importin 13 (IPO13) is a unique member, with a critical role in early embryonic development through nuclear transport of key regulators, such as transcription factors Pax6, Pax3, and ARX. Here, we examined the role of IPO13 in neuronal differentiation for the first time, using a mouse embryonic stem cell (ESC) model and a monolayer-based differentiation protocol to compare IPO13^−/− to wild type ESCs. Although IPO13^−/− ESCs differentiated into neural progenitor cells, as indicated by the expression of dorsal forebrain progenitor markers, reduced expression of progenitor markers Pax6 and Nestin compared to IPO13^−/− was evident, concomitant with reduced nuclear localisation/transcriptional function of IPO13 import cargo Pax6. Differentiation of IPO13^−/− cells into neurons appeared to be strongly impaired, as evidenced by altered morphology, reduced expression of key neuronal markers, and altered response to the neurotransmitter glutamate. Our findings establish that IPO13 has a key role in ESC neuronal differentiation, in part through the nuclear transport of Pax6.

Pax6 is required intrinsically by thalamic progenitors for the normal molecular patterning of thalamic neurons but not the growth and guidance of their axons

BACKGROUND

In mouse embryos, the Pax6 transcription factor is expressed in the progenitors of thalamic neurons but not in thalamic neurons themselves. Its null-mutation causes early mis-patterning of thalamic progenitors. It is known that thalamic neurons generated by Pax6 (-/-) progenitors do not develop their normal connections with the cortex, but it is not clear why. We investigated the extent to which defects intrinsic to the thalamus are responsible.

RESULTS

We first confirmed that, in constitutive Pax6 (-/-) mutants, the axons of thalamic neurons fail to enter the telencephalon and, instead, many of them take an abnormal path to the hypothalamus, whose expression of Slits would normally repel them. We found that thalamic neurons show reduced expression of the Slit receptor Robo2 in Pax6 (-/-) mutants, which might enhance the ability of their axons to enter the hypothalamus. Remarkably, however, in chimeras comprising a mixture of Pax6 (-/-) and Pax6 (+/+) cells, Pax6 (-/-) thalamic neurons are able to generate axons that exit the diencephalon, take normal trajectories through the telencephalon and avoid the hypothalamus. This occurs despite abnormalities in their molecular patterning (they express Nkx2.2, unlike normal thalamic neurons) and their reduced expression of Robo2. In conditional mutants, acute deletion of Pax6 from the forebrain at the time when thalamic axons are starting to grow does not prevent the development of the thalamocortical tract, suggesting that earlier extra-thalamic patterning and /or morphological defects are the main cause of thalamocortical tract failure in Pax6 (-/-) constitutive mutants.

CONCLUSIONS

Our results indicate that Pax6 is required by thalamic progenitors for the normal molecular patterning of the thalamic neurons that they generate but thalamic neurons do not need normal Pax6-dependent patterning to become competent to grow axons that can be guided appropriately.

Lessons from mouse chimaera experiments with a reiterated transgene marker: revised marker criteria and a review of chimaera markers

Recent reports of a new generation of ubiquitous transgenic chimaera markers prompted us to consider the criteria used to evaluate new chimaera markers and develop more objective assessment methods. To investigate this experimentally we used several series of fetal and adult chimaeras, carrying an older, multi-copy transgenic marker. We used two additional independent markers and objective, quantitative criteria for cell selection and cell mixing to investigate quantitative and spatial aspects of developmental neutrality. We also suggest how the quantitative analysis we used could be simplified for future use with other markers. As a result, we recommend a five-step procedure for investigators to evaluate new chimaera markers based partly on criteria proposed previously but with a greater emphasis on examining the developmental neutrality of prospective new markers. These five steps comprise (1) review of published information, (2) evaluation of marker detection, (3) genetic crosses to check for effects on viability and growth, (4) comparisons of chimaeras with and without the marker and (5) analysis of chimaeras with both cell populations labelled. Finally, we review a number of different chimaera markers and evaluate them using the extended set of criteria. These comparisons indicate that, although the new generation of ubiquitous fluorescent markers are the best of those currently available and fulfil most of the criteria required of a chimaera marker, further work is required to determine whether they are developmentally neutral.

Visualized gene network reveals the novel target transcripts Sox2 and Pax6 of neuronal development in trans-placental exposure to bisphenol A

Background

Bisphenol A (BPA) is a ubiquitous endocrine disrupting chemical in our daily life, and its health effect in response to prenatal exposure is still controversial. Early-life BPA exposure may impact brain development and contribute to childhood neurological disorders. The aim of the present study was to investigate molecular target genes of neuronal development in trans-placental exposure to BPA.

Methodology

A meta-analysis of three public microarray datasets was performed to screen for differentially expressed genes (DEGs) in exposure to BPA. The candidate genes of neuronal development were identified from gene ontology analysis in a reconstructed neuronal sub-network, and their gene expressions were determined using real-time PCR in 20 umbilical cord blood samples dichotomized into high and low BPA level groups upon the median 16.8 nM.

Principal Findings

Among 36 neuronal transcripts sorted from DAVID ontology clusters of 457 DEGs using the analysis of Bioconductor limma package, we found two neuronal genes, sex determining region Y-box 2 (Sox2) and paired box 6 (Pax6), had preferentially down-regulated expression (Bonferroni correction p-value <10⁻⁴ and log2-transformed fold change ≤−1.2) in response to BPA exposure. Fetal cord blood samples had the obviously attenuated gene expression of Sox2 and Pax6 in high BPA group referred to low BPA group. Visualized gene network of Cytoscape analysis showed that Sox2 and Pax6 which were contributed to neural precursor cell proliferation and neuronal differentiation might be down-regulated through sonic hedgehog (Shh), vascular endothelial growth factor A (VEGFA) and Notch signaling.

Conclusions

These results indicated that trans-placental BPA exposure down-regulated gene expression of Sox2 and Pax6 potentially underlying the adverse effect on childhood neuronal development.

PAX6 downregulates miR-124 expression to promote cell migration during embryonic stem cell differentiation

PAX6-null mice exhibit defects in multiple organs leading to neonatal lethality, but the mechanism by which this occurs has not yet fully elucidated. In this study, we generated induced pluripotent stem cells (iPSCs) from Pax6-mutant mice and investigated the effect of PAX6 on cell fate during embryoid body (EB) formation. We found that PAX6 promotes cell migration by directly downregulating miR-124, which is important for the fate transition of migratory cells during gastrulation of embryonic stem (ES) cells. Although several downstream targets of miR-124 have been reported, little is known regarding the upstream regulation of miR-124. When we observed EB formation of iPSCs from Pax6-mutant mice, we found that higher levels of miR-124 in Pax6 homozygous EBs (Homo-EBs) inhibited cell migration, whereas inhibition of miR-124 in Homo-EBs rescued the migratory phenotypes associated with PAX6 deficiency. Further, we found that PAX6 binds to the promoter regions of the miR-124-3 gene and directly represses its expression. Therefore, we propose a novel PAX6-miR-124 pathway that controls ES cell migration. Our findings may provide important information for studies on ES cell differentiation and embryonic development.

Brief report: miR-290-295 regulate embryonic stem cell differentiation propensities by repressing Pax6

microRNAs of the miR-290-295 family are selectively expressed at high levels in mouse embryonic stem cells (mESCs) and have established roles in regulating self-renewal. However, the potential influence of these microRNAs on cell fate acquisition during differentiation has been overlooked. Here, we show that miR-290-295 regulate the propensity of mESCs to acquire specific fates. We generated a new miR-290-295-null mESC model, which exhibits increased propensity to generate ectoderm, at the expense of endoderm and mesoderm lineages. We further found that in wild-type cells, miR-290-295 repress Pax6 and ectoderm differentiation; accordingly, Pax6 knockdown partially rescues the mESCs differentiation impairment that is caused by loss of miR-290-295. Thus, in addition to regulating self-renewal, the large reservoir of miR-290-295 in undifferentiated mESCs fine-tunes the expression of master transcriptional factors, such as Pax6, thereby regulating the equilibrium of fate acquisition by mESC descendants.