The Biological Function And Molecular Mechanism Of TGF-Î'eta Superfamily Member Dvr1 Gene During Zebrafish Gastrulation

Gastrulation is a pivotal phase of vertebrate development during which the body plan is established via a series complex of morphogenetic movements. Vertebrate gastrulation consists of three evolutionarily conserved morphogenetic processes: epiboly, internalization of presumptive mesendoderm, and convergence and extension(C&E). In zebrafish gastrulation, C&E movements narrow the germ layers mediolaterally and elongate them anteroposteriorly to sculpt the body plan.Recent work has implicated that non-canonical Wnt/PCP signaling is the key regulator of C&E movements during vertebrate gastrulation. The core members of the non-canonical Wnt/PCP pathway that have been shown to regulate C&E include wnt5, wnt11, fzd2, fzd7, dvl, vangl2, glypican4 and prickle1. Current evidence indicates that, in addition to the Wnt/PCP pathway, other pathways, like those involved in chemotaxis, cell-cell and cell-extracellular matrix adhesion, cell fate specification, also regulate C&E movements.The transforming growth factor beta(TGF-β) superfamily is a large family of structurally and functionally related polypeptide growth factors and have been categorized into severally major subfamilies, including TGF-β proteins, Bone Morphogenetic Proteins(BMPs), Growth Differentiation Factors(GDFs), Glial-derived Neurotrophic Factors(GDNFs), Activins, Nodal, Lefty and so on. Although important functions of TGF-β signaling pathway in both the developing embryo and the adult organism including cell growth, cell differentiation, apoptosis, cellular homeostasis, particularly in germ layers formation and body axis patterning have been identified, the mechanisms that control C&E movements during vertebrate gastrulation are less well understood. Until now, it is generally known that bmp2 b is required for the specification of ventrolateral cell fates during embryonic dorsoventral patterning. And bmp2 b is also the only one gene which participates in proper C&E movements in many members of TGF-β superfamily. In our study, this is the first time to report that another member of TGF-β superfamily, the dvr1 gene, a zebrafish homologue of Gdf subfamiliy, also regulates C&E movements during zebrafish gastrulation.Zebrafish dvr1 is a maternal gene and widely expressed with high level during cleavage and blastula period. As gastrulation proceeds, the expression is reduced and also can be detected in a small number of cells which closely resemble the primordial germ cells. Ubiquitous expression of dvr1 continues until late gastrulation stage but expression gets more restricted in the tail-bud.To determine the biological function of dvr1 during embryonic development, we take advantage of morpholino technology which has been shown to specifically silence gene functions in zebrafish. Two non-overlapping morpholinos are designed and injection of either morpholino into embryos causes severe C&E movements defects, resulting in wider dorsal structure and shortened anterior-positeror body axis. Our previous work shows that setdb2, a SET domain-containing protein possessing potential histone H3K9 methyltransferase activity, regulates left-right asymmetry. Recent data suggests that similar C&E defects are also observed in setdb2 morphant embryos combined with up-regulation of dvr1 mRNA level. Furthmore, dvr1 knockdown is sufficient to rescue setdb2 morpholino-induced C&E movements defects. Meanwhile, there is a strong negative correlation between dvr1 and setdb2 gene transcriptional pattern and setdb2 morpholino can induce the expression of dvr1 in blastula stage injection. Taken together, the results presented here indicate a novel requirement for dvr1 gene in the regulation of C&E movements during zebrafish gastrulation, especially in the convergence of the lateral-dorasal and dorsal domain, and in the extension of dorsal middle-posterior domain which undergo massive cell intercalation movements. Particularly, dvr1 is found to be a bona fide downstream target of setdb2 which may regulate the zygotic expression of dvr1 to control C&E movements during embryonic development.