Depletion Of Suppressor Of Cytokine Signaling-1a Causes Hepatic Steatosis And Insulin Resistance In Zebrafish

The JAK/STAT signaling pathway, which is one of the core transmission systems that mediate the extracellular signals for the activation of many key signal cascades involved in various physiological processes. More recently, members from a small protein family identified as suppressors of cytokine signaling (SOCS) have been found to form negative feedback loops to inhibit JAK/STAT signaling.SOCSprotein is a kind of negative feedback regulating protein molecules, which participate in the signaling pathways mediated by cytokines. Based on mouse genetic studies, distinguishable physiological roles for several members of the SOCS family have been delineated. Among the mouse SOCS proteins, SOCS1has been shown to be mainly involved in IFNy responses and prolactin signaling, whereas SOCS3plays regulatory roles in leukemia inhibitory factor and IL-6signaling and placental development. The observation of gigantism in SOCS2-deficient mice provides clear evidence that SOCS2is a negative regulator of GH signaling. Moreover, there is a growing body of evidence suggesting a role for SOCS proteins in insulin signaling.Somatic growth is a tightly regulated process that is dependent on GH signaling and action. Related to its role on postnatal growth promotion, GH has many other potent effects, including lipid, glucose, and mineral metabolism. In mammals, the activation of JAK2/STAT5signaling via GH leads to the transcriptional activation of a wide range of target genes, including IGF-I, anti-apoptotic genes, and SOCS2. Insulin is a peptide hormone produced by β-cells in the pancreas. Its main function is to promote the absorption of glucose from the blood to other tissues and the storage of fat for use as energy. Normally, GH also influences insulin signaling by antagonizing the action of insulin on glucose and lipid homeostasis in diverse tissues. In mammal animal models, chronic GH excess may promote insulin resistance by increasing hepatic glucose production and triglyceride storage.The zebrafish SOCS family contains at least12members. Zebrafish socs1a has been found to exhibit significant expression in the liver. The expression of zebrafish socsla may be induced by LPS and poly I:C, suggesting its potential roles as an inhibitor of IFN signaling. However, it has also been reported that socs1a may also be stimulated significantly in the livers of GHor GHR overexpressing transgenic zebrafish, indicating its involvement in teleost GH signaling pathways. Utilizing a dozen of powerful genetically modified mouse models, the physiological functions of each SOCS family member have been carefully defined. However, there is no similar genetic model for non-mammalian species. The specificity within the SOCS family of proteins in teleost model has not yet been elucidated.To address these questions, we depleted SOCS1a using the transcription activator-like (TAL) effector nucleases (TALENs) technique to understand its physiological roles in zebrafish.Although elevated levels of JAK/STAT5activation and erythropoiesis have been observed in socs1a-deficient zebrafish, these animals exhibited normal growth during the early stages. An upregulated expression level of gatal was found in socs1a-/-embryoscompared with wild-type embryos at24hpf. A modest increase in the numbers of erythrocytes was observed in socs1a-/-adults compared with wild-type adults, as determined through the red blood cell counts at90dpf. There are no significant variations in the expression levels of the typical inflammatory cytokines, such as IFN-y, IL-1, IL-6, and TNFa, between the wild-type control and their socs1a-deficient siblings. Socs1a-deficient zebrafish began to grow slowly with certain mortalities after20days post fertilization (dpf), while the heterozygous socs1a-deficient zebrafish exhibited enhanced somatic growth.Decreased adiposity, hepatic steatosis, and insulin resistance were observed in our socs1a-deficient adult zebrafish, which is similar to the lipodystrophy phenotypes observed in mamals. Comparative transcriptomic analyses revealed elevated levels of gluconeogenesis, lipolysis and hypoxia-inducible response and decreased activities of lipogenesis and glycolysis in the hepatocytes of socsla-deflicient adult zebrafish. Evident mitochondrial dysfunction has also been observed in hepatocytes from socs1a-deficient zebrafish.Taken together, our results suggest that the negative regulatory roles of SOCS1a on JAK/STAT5signaling may be involved in the suppression of the erythropoiesis and growth hormone activities, which was also reflected with the fact of the enhanced somatic growth performance observed in the heterozygous socs1a-deficient fish. The differences in the effects caused by SOCS1a depletion on insulin sensitivity, lipid metabolism and inflammatory responses between zebrafish and mammalian models observed here may reflect differences between the functional mechanisms of SOCS members in terrestrial mammals and aquatic teleosts.