Foxd4l1.1 negatively regulates transcription of neural repressor ventx1.1 during neuroectoderm formation in Xenopus embryos

Abstract

Neuroectoderm formation is the first step in development of a proper nervous system for vertebrates. The developmental decision to form a non-neural ectoderm versus a neural one involves the regulation of BMP signaling, first reported many decades ago. However, the precise regulatory mechanism by which this is accomplished has not been fully elucidated, particularly for transcriptional regulation of certain key transcription factors. BMP4 inhibition is a required step in eliciting neuroectoderm from ectoderm and Foxd4l1.1 is one of the earliest neural genes highly expressed in the neuroectoderm and conserved across vertebrates, including humans. In this work, we focused on how Foxd4l1.1 downregulates the neural repressive pathway. Foxd4l1.1 inhibited BMP4/Smad1 signaling and triggered neuroectoderm formation in animal cap explants of Xenopus embryos. Foxd4l1.1 directly bound within the promoter of endogenous neural repressor ventx1.1 and inhibited ventx1.1 transcription. Foxd4l1.1 also physically interacted with Xbra in the nucleus and inhibited Xbra-induced ventx1.1 transcription. In addition, Foxd4l1.1 also reduced nuclear localization of Smad1 to inhibit Smad1-mediated ventx1.1 transcription. Foxd4l1.1 reduced the direct binding of Xbra and Smad1 on ventx1.1 promoter regions to block Xbra/Smad1-induced synergistic activation of ventx1.1 transcription. Collectively, Foxd4l1.1 negatively regulates transcription of a neural repressor ventx1.1 by multiple mechanisms in its exclusively occupied territory of neuroectoderm, and thus leading to primary neurogenesis. In conjunction with the results of our previous findings that ventx1.1 directly represses foxd4l1.1, the reciprocal repression of ventx1.1 and foxd4l1.1 is significant in at least in part specifying the mechanism for the non-neural versus neural ectoderm fate determination in Xenopus embryos.

Foxd4l1.1 Negatively Regulates Chordin Transcription in Neuroectoderm of Xenopus Gastrula

“…[13]. In addition, Foxd4l1.1 promotes inhibitory phosphorylation at the linker region of Smad1 and reduces Smad1 C-terminal activatory phosphorylation [13]. These results point to Foxd4l1.1 inhibiting target gene transcription at multiple levels.…”

“…Foxd4l1.1 (also known as Foxd5a/b) is an immediate-early neural marker whose expression begins under the BMP inhibited condition in the prospective neuroectoderm, which is necessary for maintaining neural stemness and neural induction [11,12]. We have shown that Foxd4l1.1 directly inhibits the ectodermal fate by binding to FRE (Foxd4l1.1 response element) within the ventx1.1 promoter region [13]. Foxd4l1.1 physically interacts with Xbra, and it blocks Xbra interacting with Smad1 to interact with and activate ventx1.1 transcription.…”

“…Foxd4l1.1 physically interacts with Xbra, and it blocks Xbra interacting with Smad1 to interact with and activate ventx1.1 transcription. [13]. In addition, Foxd4l1.1 promotes inhibitory phosphorylation at the linker region of Smad1 and reduces Smad1 C-terminal activatory phosphorylation [13].…”

Functional Roles of FGF Signaling in Early Development of Vertebrate Embryos

“…More recently, Foxd4l1.1, an immediate early neural marker, was shown to be induced following inhibition of BMP signaling [153]. Foxd4l1.1 in naive neuroectoderm can induce expression of fgf8a/b in animal explants without xbra and fgf4 expression [153]. FGF8b contains Smad1 inactivation activity via linker region phosphorylation.…”

“…The requirement of an FGF signal in addition to the inhibition of BMP for neuroectoderm specification remains to be clarified to resolve the long controversy between the neural induction and default neurogenesis models. In addition, FGF2, FGF4, FGF8, and Foxd4l1.1 may have different neural target genes but they share remarkable similarity in inhibiting BMP signaling for R-Smad(s) by generating inhibitory phosphorylation of the Smad linker region [55,123,139,149,153,158,160]. Collectively, FGF signaling is important for modulating BMP signaling in neural induction; however, additional details for various FGF ligands and their FGFRs and their individual mode of action are needed for a more complete picture to emerge.…”