Functional Study Of Ascl1 In The Early Development Of Xenopus Embryo

Germ layer specification is one of the key events during early embryogenesis.Deciphering molecular mechanisms underlying the germ layer specification has been one of the main topics in the study of developmental biology for several decades.Xenopus laevis has been a well-established model system for delineating the function of maternal factors in the germ layer specification.VegT is the maternal determinant of mesendoderm formation in Xenopus.VegT achieves its essential function through transactivating the nodal-related genes and a panel of evolutionarily conserved mesendoderm transcription factors.The restricted activity of VegT in the vegetal hemisphere is essential for establishing the normal germ layer pattern along the animal-vegetal axis.Wnt11 is a component of the maternal dorsal determinant(s)that is required for the formation of Spemann's organizer and the differentiation of all three germ layers along the dorsal ventral axis.The uncommitted ectoderm in Xenopus is pluripotent and can be directed into mesoderm or endoderm by various growth factors in vitro,while ectoderm will differentiate to nervous system and ectodermis in vivo.Ectoderm cells have an intrinsic propensity to adopt neural fates.It has been mysterious of how the neural tendency of ectoderm cells is regulated.It also remains unclear how the activities of vegetal inducing signals are restricted in the vegetal hemisphere during the primary induction.Ascl1 is orthologous to the members of Drosophila achaete-scute complex.Studies in invertebrates and vertebrates have revealed essential functions for Ascl1 in the neuronal specification and differentiation in central and peripheral nervous systems.Ascl1 is also able to convert non-neural somatic cells into functional neurons.Therefore Ascl1 plays an important role in neural development.It has been reported that Ascl1 is a maternal gene in Xenopus,but little is known about its maternal function.Here,we performed a series of gain-and loss-of-function experiments for the maternal Ascl1 and uncovered its essential role in the germ layer specification.First,we found that abnormal expression of Ascl1 causes a delay of gastrulation,defects of germ layer specification and a shortened body axis during early Xenopus development.Second,we demonstrated that Ascl1 is a repressor of mesedoderm induced by VegT butnot of Nodal-induced mesendoderm.Third,we proved that the N-terminal domain of Ascl1 interacts with HDAC1 and therefore Ascl1 represses expression of the mesendoderm genes depending on HDAC activity.Ascl1 prevents the deposition of acetylated histone marks in the promoter regions of VegT targets.In addition,we found that the N-and C-terminal domains of Ascl1 have distinct activities in regulating of germ layer specification.Finally,we demonstrated that maternal Ascl1 is required for neural induction,neural progenitor maintenance and neural differentiation.Our study establishes a key role for the maternal Asc1 l in germ layer specification and neurogenesis.Ascl1 regulates mesendoderm formation via HDAC1-dependent antagonism of VegT and is essential for neural development.The data presented here significantly extended our understanding of early embryonic pattern formation.