The germ cell nuclear factor is required for retinoic acid signaling during Xenopus development

The germ cell nuclear factor (GCNF, NR6A1) is a nuclear orphan receptor that functions as a transcriptional repressor and is transiently expressed in mammalian carcinoma cells during retinoic acid (RA) induced neuronal differentiation. During Xenopus laevis development, the spatiotemporal expression pattern of embryonic GCNF (xEmGCNF) suggests a role in anteroposterior specification of the neuroectoderm. Here, we show that RA treatment of Xenopus embryos enhances xEmGCNF expression. Moreover, we present evidence for the relevance of this finding in the context of primary neurogenesis and hindbrain development. During early development of the central nervous system, RA signals promote posterior transformation of the neuroectoderm and increase the number of cells undergoing primary neurogenesis. Our loss-of-function analyses using a xEmGCNF-specific morpholino antisense oligonucleotide indicate that xEmGCNF is required for the effect of RA on primary neurogenesis. This may be caused by transcriptional regulation of the gene encoding the RA-degrading enzyme CYP26, since this gene is derepressed after depletion of xEmGCNF and an antimorph of xEmGCNF directly activates transcription of CYP26, also in absence of protein synthesis. The effect of xEmGCNF knockdown on hindbrain patterning is similar to conditions of reduced RA signaling, which may be caused by a reduction of RAR gamma expression specifically in the presumptive hindbrain.