RESUMO
We analyzed the influence of presenilins on the genetic cascades that control neuronal differentiation in Xenopus embryos. Resembling sonic hedgehog (shh) overexpression, presenilin mRNA injection reduced the number of N-tubulin+ primary neurons and modulated Gli3 and Zic2 according to their roles in activating and repressing primary neurogenesis, respectively. Presenilin increased shh expression within its normal domain, mainly in the floor plate, whereas an antisense X-presenilin-alpha morpholino oligonucleotide reduced shh expression. Both shh and presenilin promoted cell proliferation and apoptosis, but the effects of shh were widely distributed, while those resulting from presenilin injection coincided with the range of shh signaling. We suggest that presenilin may modulate primary neurogenesis, proliferation, and apoptosis in the neural plate, through the enhancement of shh signaling.
Assuntos
Proteínas de Membrana/genética , Proteínas do Tecido Nervoso , Neurônios/citologia , Proteínas Repressoras , Transativadores/genética , Proteínas de Xenopus , Xenopus laevis/embriologia , Secretases da Proteína Precursora do Amiloide , Animais , Apoptose , Ácido Aspártico Endopeptidases , Diferenciação Celular , Divisão Celular , Sistema Nervoso Central/embriologia , Proteínas de Ligação a DNA/genética , Regulação para Baixo , Embrião não Mamífero/citologia , Embrião não Mamífero/metabolismo , Endopeptidases/metabolismo , Regulação da Expressão Gênica no Desenvolvimento , Proteínas Hedgehog , Hibridização In Situ , Fatores de Transcrição Kruppel-Like , Proteínas de Membrana/fisiologia , Mutagênese Sítio-Dirigida , Oligonucleotídeos Antissenso , Presenilina-1 , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transdução de Sinais , Transativadores/fisiologia , Fatores de Transcrição/genética , Tretinoína/farmacologia , Tubulina (Proteína)/genética , Tubulina (Proteína)/metabolismo , Xenopus laevis/genética , Xenopus laevis/metabolismo , Proteína Gli3 com Dedos de ZincoRESUMO
Previous work has shown that the posteriorising agent retinoic acid can accelerate anterior neuronal differentiation in Xenopus laevis embryos (Papalopulu, N. and Kintner, C. (1996) Development 122, 3409-3418). To elucidate the role of retinoic acid in the primary neurogenesis cascade, we investigated whether retinoic acid treatment of whole embryos could change the spatial expression of a set of genes known to be involved in neurogenesis. We show that retinoic acid expands the N-tubulin, X-ngnr-1, X-MyT1, X-&Dgr;-1 and Gli3 domains and inhibits the expression of Zic2 and sonic hedgehog in the neural ectoderm, whereas a retinoid antagonist produces opposite changes. In contrast, sonic and banded hedgehog overexpression reduced the N-tubulin stripes, enlarged the neural plate at the expense of the neural crest, downregulated Gli3 and upregulated Zic2. Thus, retinoic acid and hedgehog signaling have opposite effects on the prepattern genes Gli3 and Zic2 and on other genes acting downstream in the neurogenesis cascade. In addition, retinoic acid cannot rescue the inhibitory effect of Notch(ICD), Zic2 or sonic hedgehog on primary neurogenesis. Our results suggest that retinoic acid acts very early, upstream of sonic hedgehog, and we propose a model for regulation of differentiation and proliferation in the neural plate, showing that retinoic acid might be activating primary neurogenesis by repressing sonic hedgehog expression.
