The SPRY domain of SSB-2 adopts a novel fold that presents conserved Par-4-binding residues

The four mammalian SPRY domain-containing SOCS box proteins (SSB-1 to SSB-4) are characterized by a C-terminal SOCS box and a central SPRY domain. We have determined the first SPRY-domain structure, as part of SSB-2, by NMR. This domain adopts a novel fold consisting of a beta-sandwich structure formed by two four-stranded antiparallel beta-sheets with a unique topology. We demonstrate that SSB-1, SSB-2 and SSB-4, but not SSB-3, bind prostate apoptosis response protein-4 (Par-4). Mutational analysis of SSB-2 loop regions identified conserved structural determinants for its interaction with Par-4 and the hepatocyte growth factor receptor, c-Met. Mutations in analogous loop regions of pyrin and midline-1 SPRY domains have been shown to cause Mediterranean fever and Opitz syndrome, respectively. Our findings provide a template for SPRY-domain structure and an insight into the mechanism of SPRY-protein interaction.     

The SPRY domain-containing SOCS box protein 1 (SSB-1) interacts with MET and enhances the hepatocyte growth factor-induced Erk-Elk-1-serum response element pathway

The suppressor of cytokine signaling (SOCS) protein family includes a SPRY (repeats in splA and RyR) domain-containing SOCS box protein (SSB) subfamily, which consists of four members, SSB-1, SSB-2, SSB-3, and SSB-4. These proteins contain a central SPRY domain and a C-terminal SOCS box. Although some of the SOCS protein subfamilies function as adaptors for a large family of ubiquitin-protein isopeptide ligases to regulate certain signaling pathways, the function of the SSB subfamily remains to be determined. In our previous studies, we have found that two SPRY domain-containing proteins, RanBP9 and RanBP10, interact with MET through the SPRY domain. In the present study, we explored the function of SSB proteins in the regulation of the hepatocyte growth factor (HGF)-MET signaling. Our results showed that all four SSB proteins also interacted with the MET. The MET interaction with SSB-1 was further investigated. We demonstrated that SSB-1 bound to MET tyrosine kinase domain through its SPRY domain. MET interacted with SSB-1 in both the absence and the presence of HGF, but HGF treatment resulted in the recruitment of more SSB-1 by MET. We showed that overexpression of SSB-1 but not other SSB proteins enhanced the HGF-induced serum response element (SRE)-luciferase activity. Overexpression of SSB-1 exhibited no effect on the basal level or epidermal growth factor-induced SRE-luciferase activity. SSB-1 also enhanced HGF-induced Erk phosphorylation. Suppression of SSB-1 by the RNA interference method down-regulated HGF-induced SRE-luciferase activity and decreased Elk-1 activation. These results suggest that SSB-1 may play an important role in enhancing the HGF-induced Erk-Elk-1-SRE pathway. Furthermore, we demonstrated that in response to HGF stimulation, the SSB-1 protein became phosphorylated at tyrosine residue 31. The phosphorylated SSB-1 protein bound to p120Ras-GTPase-activating protein (GAP) but did not promote the degradation of p120RasGAP, indicating that enhanced HGF-MET signaling by overexpression of SSB-1 was not dependent on p120RasGAP degradation.     

Genetic deletion of murine SPRY domain-containing SOCS box protein 2 (SSB-2) results in very mild thrombocytopenia

The SSB family is comprised of four highly homologous proteins containing a C-terminal SOCS box motif and a central SPRY domain. No function has yet been ascribed to any member of this family in mammalian species despite a clear role for other SOCS proteins in negative regulation of cytokine signaling. To investigate its physiological role, the murine Ssb-2 gene was deleted by homologous recombination. SSB-2-deficient mice were shown to have a reduced rate of platelet production, resulting in very mild thrombocytopenia (25% decrease in circulating platelets). Tissue histology and other hematological parameters were normal, as was the majority of serum biochemistry, with the exception that blood urea nitrogen (BUN) levels were decreased in mice lacking SSB-2. Quantitative analysis of SSB mRNA levels indicated that SSB-1, -2, and -3 were ubiquitously expressed; however, SSB-4 was only expressed at very low levels. SSB-2 expression was observed in the kidney and in megakaryocytes, a finding consistent with the phenotype of mice lacking this gene. Deletion of SSB-2 thus perturbs the steady-state level of two tightly controlled homeostatic parameters and identifies a critical role for SSB-2 in regulating platelet production and BUN levels.     

SPRY Domain-Containing SOCS Box Protein 2: Crystal Structure and Residues Critical for Protein Binding

The four mammalian SPRY (a sequence repeat in dual-specificity kinase splA and ryanodine receptors) domain-containing suppressor of cytokine signalling (SOCS) box proteins (SSB-1 to -4) are characterised by a C-terminal SOCS box and a central SPRY domain. The latter is a protein interaction module found in over 1600 proteins, with more than 70 encoded in the human genome. Here we report the crystal structure of the SPRY domain of murine SSB-2 and compare it with the SSB-2 solution structure and crystal structures of other B30.2/SPRY domain-containing family proteins. The structure is a bent β-sandwich, consisting of two seven-stranded β-sheets wrapped around a long loop that extends from the centre strands of the inner or concave β-sheet; it closely matches those of GUSTAVUS and SSB-4. The structure is also similar to those of two recently determined Neuralized homology repeat (NHR) domains (also known as NEUZ domains), with detailed comparisons, suggesting that the NEUZ/NHR domains form a subclass of SPRY domains. The binding site on SSB-2 for the prostate apoptosis response-4 (Par-4) protein has been mapped in finer detail using mutational analyses. Moreover, SSB-1 was shown to have a Par-4 binding surface similar to that identified for SSB-2. Structural perturbations of SSB-2 induced by mutations affecting its interaction with Par-4 and/or c-Met have been characterised by NMR. These comparisons, in conjunction with previously published dynamics data from NMR relaxation studies and coarse-grained dynamics simulation using normal mode analysis, further refine our understanding of the structural basis for protein recognition of SPRY domain-containing proteins.     

Distinct developmental expression of two elongase family members in zebrafish

Despite the known importance of long-chained polyunsaturated fatty acids (LC-PUFA) during development, very little is known about their utilization and biosynthesis during embryogenesis. Combining the advantages of the existence of a complete range of enzymes required for LC-PUFA biosynthesis and the well established developmental biology tools in zebrafish, we examined the expression patterns of three LC-PUFA biosynthesis genes, Elovl2-like elongase (elovl2), Elovl5-like elongase (elovl5) and fatty acyl desaturase (fad) in different zebrafish developmental stages. The presence of all three genes in the brain as early as 24 hours post fertilization (hpf) implies LC-PUFA synthesis activity in the embryonic brain. This expression eventually subsides from 72 hpf onwards, coinciding with the initiation of elovl2 and fad expression in the liver and intestine, 2 organs known to be involved in adult fish LC-PUFA biosynthesis. Collectively, these patterns strongly suggest the necessity for localized production of LC-PUFA in the brain during in early stage embryos prior to the maturation of the liver and intestine. Interestingly, we also showed a specific expression of elovl5 in the proximal convoluted tubule (PCT) of the zebrafish pronephros, suggesting a possible new role for LC-PUFA in kidney development and function.     

Calycosin Promotes Angiogenesis Involving Estrogen Receptor and Mitogen-Activated Protein Kinase (MAPK) Signaling Pathway in Zebrafish and HUVEC

Background

Angiogenesis plays an important role in a wide range of physiological processes, and many diseases are associated with the dysregulation of angiogenesis. Radix Astragali is a Chinese medicinal herb commonly used for treating cardiovascular disorders and has been shown to possess angiogenic effect in previous studies but its active constituent and underlying mechanism remain unclear. The present study investigates the angiogenic effects of calycosin, a major isoflavonoid isolated from Radix Astragali, in vitro and in vivo.

Methodology

Tg(fli1:EGFP) and Tg(fli1:nEGFP) transgenic zebrafish embryos were treated with different concentrations of calycosin (10, 30, 100 µM) from 72 hpf to 96 hpf prior morphological observation and angiogenesis phenotypes assessment. Zebrafish embryos were exposed to calycosin (10, 100 µM) from 72 hpf to 78 hpf before gene-expression analysis. The effects of VEGFR tyrosine kinase inhibitor on calycosin-induced angiogenesis were studied using 72 hpf Tg(fli1:EGFP) and Tg(fli1:nEGFP) zebrafish embryos. The pro-angiogenic effects of calycosin were compared with raloxifene and tamoxifen in 72 hpf Tg(fli1:EGFP) zebrafish embryos. The binding affinities of calycosin to estrogen receptors (ERs) were evaluated by cell-free and cell-based estrogen receptor binding assays. Human umbilical vein endothelial cell cultures (HUVEC) were pretreated with different concentrations of calycosin (3, 10, 30, 100 µM) for 48 h then tested for cell viability and tube formation. The role of MAPK signaling in calycosin-induced angiogenesis was evaluated using western blotting.

Conclusion

Calycosin was shown to induce angiogenesis in human umbilical vein endothelial cell cultures (HUVEC) in vitro and zebrafish embryos in vivo via the up-regulation of vascular endothelial growth factor (VEGF), VEGFR1 and VEGFR2 mRNA expression. It was demonstrated that calycosin acted similar to other selective estrogen receptor modulators (SERMs), such as raloxifene and tamoxifen, by displaying selective potency and affinity to estrogen receptors ERα and ERβ. Our results further indicated that calycosin promotes angiogenesis via activation of MAPK with the involvement of ERK1/2 and ER. Together, this study revealed, for the first time, that calycosin acts as a selective estrogen receptor modulator (SERM) to promote angiogenesis, at least in part through VEGF-VEGFR2 and MAPK signaling pathways.     

Zebrafish Jak2a plays a crucial role in definitive hematopoiesis and blood vessel formation

We examined the role of zebrafish (Danio rerio) Jak2a, a homolog of mammalian Jak2, in the developing embryo by injecting in vitro synthesized Jak2a shRNA into zebrafish zygotes. Blood circulation was suppressed in Jak2a shRNA-injected embryos from 24 hours post fertilization (hpf) and all embryos died with enlarged pericardium, shortened body lengths, and defects in some vasculature within 8 days post fertilization. O-dianisidine staining of red blood cells revealed normal blood island formation with no circulating red blood cells. As in Jak2^−/− transgenic mice, expression of definitive Ba1 globin was significantly reduced in Jak2a knockdown embryos at 36 hpf, whereas expression of other hematopoietic markers, primitive be1 globin, gata-1, and scl, were unaffected. More importantly, blood vessel formation was disturbed in Jak2a knockdown embryos as revealed by alkaline phosphatase staining at 72 hpf. Thus, our data indicate that zebrafish Jak2a is important in both definitive hematopoiesis and blood vessel formation.     

Functional differentiation of bmp2a and bmp2b genes in zebrafish

Bone morphogenetic protein 2 plays an important role in the regulation of osteoblast proliferation and differentiation. Phylogenetic analysis showed that the bmp2 ortholog evolved from the same ancestral gene family in vertebrates and was duplicated in teleost, which were named bmp2a and bmp2b. The results of whole-mount in situ hybridization showed that the expression locations of bmp2a and bmp2b in zebrafish were different in different periods (24 hpf, 48 hpf, 72 hpf), which revealed potential functional differentiation between bmp2a and bmp2b. Phenotypic analysis showed that bmp2a mutations caused partial rib and vertebral deformities in zebrafish, while bmp2b^−/− embryos died massively after 12 hpf due to abnormal somite formation. We further explored the expression pattern changes of genes (bmp2a, bmp2b, smad1, fgf4, runx2b, alp) related to skeletal development at different developmental stages (20 dpf, 60 dpf, 90 dpf) in wild-type and bmp2a^−/− zebrafish. The results showed that the expression of runx2b in bmp2a^−/− was significantly downregulated at three stages and the expression of other genes were significantly downregulated at 90 dpf compared with wild-type zebrafish. The study revealed functional differentiation of bmp2a and bmp2b in zebrafish embryonic and skeletal development.     

Structural basis for protein recognition by B30.2/SPRY domains

B30.2/SPRY domains are found in numerous proteins that cover a wide spectrum of biological functions, including regulation of cytokine signaling and innate retroviral restriction. Herein, we report the crystal structure of the B30.2/SPRY domain of a SPRY domain-containing SOCS box (SSB) protein, GUSTAVUS, complexed with a 20 amino acid peptide derived from the RNA helicase VASA, revealing how these domains recognize target proteins. The peptide-binding site is conformationally rigid and has a preformed pocket. The interaction between the pocket and the Asp-Ile-Asn-Asn-Asn-Asn sequence within the peptide accounts for the high-affinity binding between GUSTAVUS and VASA. This observation led to a facile identification of the Glu-Leu-Asn-Asn-Asn-Leu sequence as the recognition motif in a proapoptotic protein Par-4 for its interaction with a GUSTAVUS homolog, SSB-1. Ensuing analyses indicated that many B30.2/SPRY domains have a similar preformed pocket, which would allow them to bind multiple targets.