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Published on: July 21, 2021
XBP1 forms a regulatory loop with BMP-4 and suppresses mesodermal and neural differentiation in Xenopus embryos
Ying Cao1, Sigrun Knöchel, Franz Oswald
1Abteilung Biochemie, Universität Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany.
Abstract:
The active form of the Xenopus X-box binding protein 1 (xXBP1) partially synergizes and partially antagonizes with BMP-4 signaling. xXBP1 overexpression inhibits mesoderm differentiation and formation of neural tissues. A functional knockdown promotes differentiation of lateral and dorsal mesoderm but not of ventral mesoderm and of neuroectoderm. We show that the active form of xXBP1 in gastrula and early neurula stage embryos is generated by removal of exon 4 and not by an endoribonuclease activity in the endoplasmic reticulum. The N-terminal region of xXBP1 which contains the basic leucine-zipper also contains a nuclear localization signal and both, the N-terminal as well as the C-terminal regions are required for xXBP1 function. The effects of xXBP1 are in part correlated to a regulatory loop between xXBP1 and BMP-4. xXBP1 and BMP-4 stimulate mutually the transcription of each other, but xXBP1 inhibits the BMP-4 target gene, Xvent-2. Both, in vitro and in vivo assays demonstrate that xXBP1 interacts with BMP-4 and Xvent-2B promoters. GST-pulldown assays reveal that xXBP1 can interact with c-Jun, the transcriptional co-activator p300 and with the BMP-4 responsive Smad1. On the other hand, xXBP1 also binds to the inhibitory Smads, Smad6 and Smad7, that can act as transcriptional co-repressors. Based on these data, we conclude that xXBP1 might function as an inhibitor of mesodermal and neural tissue formation by acting either as transcriptional activator or as repressor. This dual activity depends upon binding of co-factors being involved in the formation of distinct transcription complexes.
Insights
The active Xenopus X-box binding protein 1 (xXBP1) regulates mesoderm and neural tissue formation. It acts as both a transcriptional activator and repressor, depending on cofactor binding, influencing developmental pathways.
Area of Science:
- Developmental Biology
- Molecular Biology
- Gene Regulation
Background:
- The Xenopus X-box binding protein 1 (xXBP1) is a key transcription factor involved in embryonic development.
- Its precise role in mesoderm and neural tissue formation, particularly its interaction with Bone Morphogenetic Protein-4 (BMP-4) signaling, requires further elucidation.
Purpose of the Study:
- To investigate the function of active xXBP1 in Xenopus embryonic development.
- To determine the mechanism of active xXBP1 generation and its interaction with BMP-4 signaling pathways.
- To identify the regions of xXBP1 critical for its function and its interacting partners.
Main Methods:
- Overexpression and knockdown of xXBP1 in Xenopus embryos.
- Analysis of mesoderm and neural tissue differentiation.
- Identification of active xXBP1 processing via exon removal.
- In vitro and in vivo interaction assays (e.g., GST-pulldown) with promoter regions and protein partners.
- Analysis of target gene expression, including BMP-4 and Xvent-2.
Main Results:
- Active xXBP1 inhibits mesoderm and neural tissue formation, with knockdown promoting differentiation.
- The active form of xXBP1 is generated by exon 4 removal, not ER-associated endoribonuclease activity.
- xXBP1 exhibits a complex regulatory loop with BMP-4, mutually stimulating transcription but with xXBP1 inhibiting the BMP-4 target gene Xvent-2.
- xXBP1 interacts with BMP-4 and Xvent-2B promoters, and associates with co-activators (p300, c-Jun) and co-repressors (Smad6, Smad7).
- Both N-terminal and C-terminal regions of xXBP1 are essential for its function.
Conclusions:
- xXBP1 plays a dual role as a transcriptional activator and repressor in mesodermal and neural tissue formation.
- Its function is modulated by interactions with BMP-4 signaling components and various co-factors.
- The processing of xXBP1 and its cofactor-dependent activity provide a mechanism for regulating developmental patterning.
