Model for the regulation network of BMP/R-Smads during dorso-ventral patterning  During blastula stage, BMP signals are transduced by Smad5 proteins. Before the onset of gastrulation, BMP/Smad5 signaling activates the expression of Smad1 and Smad9, which are negatively regulated by each other and by themselves. During gastrula stage, Smad1 and Smad9 are thedirect transducersof BMP signals in promoting ventral fate.

Bone morphogenetic proteins (BMPs) are conserved morphogens that play crucial roles in dorso-ventral (DV) patterning of embryonic development and their functional deficiency leads to serious defects of ventral development. BMP signals are transduced by 3 receptor-type Smad proteins (R-Smads),Smad1,Smad5 and Smad8 (Smad9). Nevertheless, it remains largely unknown that how these R-Smads contributes to the patterning of early DV axis in a regulatory network and in a cooperative way. The previous results from loss-of-function and gain-of-function in zebrafish even led to controversial conclusions. For instance, zebrafish smad5 mutants are strongly dorsalized, whereas effective loss-of-function of smad1 does not lead to any DV patterning defects. By contrast,overexpression of smad1 instead of smad5 leads to complete rescue of zebrafish mutants of bmp2b. 

The research group led by Prof. SUN Yonghua at Institute of Hydrobiology, Chinese Academy of Sciences (IHB) identified smad9 gene from zebrafish genome and thoroughly studied the regulation network of these three R-Smads in DV patterning. The results have shown that smad9 and smad1 have a similar zygotic expression pattern, and overexpression of either smad1 or smad9 leads to ventralization, indicating that they both have intrinsic ventralizing activity. Although the smad9 or smad1 depleted embryos are not dorsalized, double depletion of smad1 and smad9 leads to strong dorsalization that cannot be rescued by smad5 overexpression.  

By contrast, overexpression of smad1 or smad9 can efficiently rescue the dorsalization of smad5 morphants, demonstrating that both smad1 and smad9 locate downstream of smad5. The transcriptional activities of smad1 and smad9 directly rely on smad5, and their transcriptional initiations are suppressed by each other. Further studies have shown that Smad5 proteins bind to the promoter regions and activate the promoter activities of smad1 and smad9, and revealed that smad1 and smad9 are the direct transcriptional targets of smad5.  

In summary, this study has clearly clarified the regulation network and cooperative roles of BMP R-Smads in zebrafish DV patterning, i.e., smad1 and smad9 act redundantly to each other and function downstream of smad5 to mediate ventral specification. Through this study, researchers have uncovered the relative roles between smad1 and smad5 in regulating zebrafish DV patterning, which has been a scientific puzzle in this field for several years. Besides, they have also found that smad1 and smad9 have overlapping functions in regulating myelopoiesis. 

The research findings were published online in the Journal of Biological Chemistry with the title "Transcriptional Factors Smad1 and Smad9 Act Redundantly to Mediate Zebrafish Ventral Specification Downstream of Smad5". This work was supported by grants from the National Basic Research Program from the Ministry of Science and Technology of China, National Science Fund for Excellent Young Scholars from the National Natural Science Foundation of China and the State Key Laboratory of Freshwater Ecology and Biotechnology.