Corepressor NCoR And SMRT Are Essential For Zebrafish A-P Axis Patterning And Primitive Myelopoiesis

Active repression of gene expression mediated by unliganded nuclear receptors plays crucial roles in early development of vertebrates. NCoR is the first identified corepressor that can repress RA inducible gene transcription in the absence of RA. Previously, NCoR was reported to be required for late-stage organogenesis in mouse but whether NCoR can affect RA-responsive early embryonic patterning is unknown. In this study, we report that knocking down ncor elevates endogenous RA signaling in zebrafish embryos and posteriorizes the neural ectoderm. Overexpressing or knocking down ncor in zebrafish embryos alters the length of hindbrain in a manner similar to decreasing or increasing RA signaling in embryos, respectively. Our results demonstrate that NCoR is essential for early hindbrain patterning by actively repressing retinoid signaling. As a close relative of NCoR, SMRT mediate target genes silencing in developmental systems and homeostasis. Previously, SMRT was reported to be required for forebrain development and anti-inflammatory signaling pathways in mouse but whether SMRT affects RA-responsive early embryonic patterning, such as A-P axis, is unknown. By knocking down smrt in zebrafish embryos, we found that unlike ncor, smrt plays no significant role in hindbrain and early neural crest patterning though it is involved in repressing retinoid signaling like ncor. Interestingly, our results showed that zebrafish SMRT is essential to the A-P specification of trunk and tail in zebrafish embryos through repressing hox gene expressions. In zebrafish, pluripotent hematopoietic stem cells generate common myeloid progenitor cells, erythroid progenitor cells and endothelial progenitor cells through progressive commitment and differentiation steps. Terminally myeloid progenitor cells differentiate to granulocyte and macrophage/monocyte lineages whereas erythroid progenitor cells differentiate to erythroid lineages. In NCoR knockout mice, definitive erythropoiesis is impaired whereas primitive erythropoiesis is normal. However, whether NCoR or/and SMRT play roles in primitive hematopoiesis in zebrafish is still unknown. By knocking down ncor or smrt in zebrafish embryos, we showed that the corepressors affect myelopoiesis differently rather than erythropoiesis. In smrt morphants, the expression of either mpx or l-plsatin, marker genes of granulocyte and macrophage/monocyte, respectively, is greatly reduced. However, in ncor morphants, only mpx expression is reduced. Analyzing smrt morphants, we demonstrated that the loss of myeloid cells in smrt-depleted embryos is not attributed to decreased proliferation or extensive apoptosis of myeloid cells, but due to affecting the specification of the earlier hematopoietic precursors and primitive myelopoietic precursors. By rescuing the phenotype in smrt morphants through forced expression of spil or scl/lmo2, we revealed that smrt acts parallel of scl/lmo2 and upstream of spil during primitive myelopoiesis. However, none of the early transcriptional factors including scl, lmo2, etsrp, hhex, gata2, draculin, c/ebpa and spil has changed their expression patterns in ncor morphants. The results suggest that NCoR is not required to the formation of myeloid progenitor but is crucial for granulogenesis. Taken together, our results demonstrated that NCoR and SMRT are essential for zebrafish A-P axis patterning and primitive myelopoiesis.