Dose-dependent nuclear beta-catenin response segregates endomesoderm along the sea star primary axis

摘要

In many invertebrates, the nuclearization of beta-catenin at one pole of the embryo initiates endomesoderm specification. An intriguing possibility is that a gradient of nuclear beta-catenin (n beta-catenin), similar to that operating in vertebrate neural tube patterning, functions to distinguish cell fates in invertebrates. To test this hypothesis, we determined the function of n beta-catenin during the early development of the sea star, which undergoes a basal deuterostomal mode of embryogenesis. We show that low levels of n beta-catenin activity initiate bra, which is expressed in the future posterior endoderm-fated territory; intermediate levels are required for expression of foxa and gata4/5/6, which are later restricted to the endoderm; and activation of ets1 and erg in the mesoderm-fated territory requires the highest n beta-catenin activity. Transcription factors acting downstream of high n beta-catenin segregate the endoderm/mesoderm boundary, which is further reinforced by Delta/Notch signaling. Significantly, therefore, in sea stars, endomesoderm segregation arises through transcriptional responses to levels of n beta-catenin activity. Here, we describe the first empirical evidence of a dose-dependent response to a dynamic spatiotemporal n beta-catenin activity that patterns cell fates along the primary axis in an invertebrate.