Targeted knockdown of insulin-like growth factor binding protein-2 disrupts cardiovascular development in zebrafish embryos

Molecular basis of sex and reproductive status in breeding zebrafish

The zebrafish ( Danio rerio) is used extensively as a model species for studies on vertebrate development and for assessing chemical effects on reproduction. Despite this, the molecular mechanisms controlling zebrafish reproduction are poorly understood. We analyzed the transcriptomic profiles of the gonads of individual zebrafish, using a 17k oligonucleotide microarray, to define the molecular basis of sex and reproductive status in sexually mature fish. The gonadal transcriptome differed substantially between sexes. Among the genes overexpressed in females, 11 biological processes were overrepresented including mitochondrion organization and biogenesis, and cell growth and/or maintenance. Among the genes overexpressed in males, six biological processes were overrepresented including protein biosynthesis and protein metabolism. Analysis of the expression of gene families known to be involved in reproduction identified a number of genes differentially expressed between ovaries and testes including a number of sox genes and genes belonging to the insulin-like growth factor and the activin-inhibin pathways. Real-time quantitative PCR confirmed the expression profiles for nine of the most differentially expressed genes and indicated that many transcripts are likely to be switched off in one of the sexes in the gonads of adult fish. Significant differences were seen between the gonad transcriptomes of individual reproductively active females reflecting their stage of maturation, whereas the testis transcriptomes were remarkably similar between individuals. In summary, we have identified molecular processes associated with (gonadal) sex specificity in breeding zebrafish and established a strong relationship between individual ovarian transcriptomes and reproductive status in females.

Insulin-like growth factor receptor 1b is required for zebrafish primordial germ cell migration and survival

Insulin-like growth factor (IGF) signaling is a critical regulator of somatic growth during fetal and adult development, primarily through its stimulatory effects on cell proliferation and survival. IGF signaling is also required for development of the reproductive system, although its precise role in this regard remains unclear. We have hypothesized that IGF signaling is required for embryonic germline development, which requires the specification and proliferation of primordial germ cells (PGCs) in an extragonadal location, followed by directed migration to the genital ridges. We tested this hypothesis using loss-of-function studies in the zebrafish embryo, which possesses two functional copies of the Type-1 IGF receptor gene (igf1ra, igf1rb). Knockdown of IGF1Rb by morpholino oligonucleotides (MO) results in mismigration and elimination of primordial germ cells (PGCs), resulting in fewer PGCs colonizing the genital ridges. In contrast, knockdown of IGF1Ra has no effect on PGC migration or number despite inducing widespread somatic cell apoptosis. Ablation of both receptors, using combined MO injections or overexpression of a dominant-negative IGF1R, yields embryos with a PGC-deficient phenotype similar to IGF1Rb knockdown. TUNEL analyses revealed that mismigrated PGCs in IGF1Rb-deficient embryos are eliminated by apoptosis; overexpression of an antiapoptotic gene (Bcl2l) rescues ectopic PGCs from apoptosis but fails to rescue migration defects. Lastly, we show that suppression of IGF signaling leads to quantitative changes in the expression of genes encoding CXCL-family chemokine ligands and receptors involved in PGC migration. Collectively, these data suggest a novel role for IGF signaling in early germline development, potentially via cross-talk with chemokine signaling pathways.

The physiological role of CTGF/CCN2 in zebrafish notochond development and biological analysis of the proximal promoter region

During mouse embryogenesis, CTGF/CCN2 is expressed in zones containing hypertrophic chondroctyes and calcifying cartilage such as long bones, ribs, vertebral column, and phalanges. But in fish, its expression is yet unclear. Development of the vertebrae is morphologically similar among vertebrates, indicating that the underlying mechanism regulating the process is highly conserved during evolution. Analysis of 3.2kb of the CTGF/CCN2 proximal promoter sequence revealed a consensus TATAA box, putative AP1, Brn-2, CdxA, C/EBP alpha, C/EBP beta, C-Ets-, delta E, HFH-2, and HSF2 binding sites. Transient expression experiments with a 5'-deletion revealed at least 4 regulatory regions in the zebrafish CTGF/CCN2 gene, 2 with a stimulatory effect on transcription and 2 with an apparent inhibitory effect after IGF-I treatment in the ZFL cell line. To study the promoter-specific expression, we constructed a series of CTGF/CCN2 (3.0-, 2.5-, 2.0-, 1.5-, 1.0-, and 0.4-kb) promoter-driven green fluorescent protein (GFP) fragments encoding the GFP cDNA transgene which was microinjected into zebrafish embryos. Morphological studies of transgenic zebrafish indicated that the CTGF/CCN2 promoter-driven GFP transcripts appeared in the notochord. Targeted knockdown of the CTGF/CCN2 gene by two antisense morpholino oligonucleotides resulted in disruptions to notochord development. From a comparative point of view, this study of the CTGF/CCN2 gene in zebrafish may correlate well with those previously published on the mouse. These molecular results suggest that CTGF/CCN2 plays an important role in notochord development and is required for general embryonic development.

Diabetic microangiopathy: IGFBP control endothelial cell growth by a common mechanism in spite of their species specificity and tissue peculiarity

Endothelial cells (EC) play a role in many diseases including diabetes mellitus. EC share common functions, such as angiogenesis and vascular remodeling both regulated by proliferation and apoptosis, anti-thrombotic properties, regulation of vascular tone, control in the passage of nutrients and secretion of peptides and growth factors. However, EC are characterized by site-specificity so their characteristics depend on the organs and tissues where they are. The IGF system induces important growth factors that control cell growth in different microvascular EC (mEC). This family includes IGF-I and IGF-II peptides, their receptors and regulatory proteins IGF-binding proteins (IGFBP-1 to IGFBP-6). The IGFBP modulate their interaction with the IGF membrane receptors and might be regulated at a transcriptional and post-transcriptional level, thus determining the biological IGF-dependent effects on target cells. The IGF system is also a mediator of vascular diseases, and its altered balance might contribute to endothelial dysfunction with the development and evolution of diabetic microangiopathy. We reported here the reviewed literature of IGFBP production from various sources of mEC, showing that they predominantly express IGFBP-2 through IGFBP-5 mRNA. The different pattern of IGFBP secretion depends on the anatomical district and on the species of the tissues. Nevertheless, based on our and other experimental observations, we suggested that a common mechanism of IGFBP regulation in mEC could be hypothized. In retinal and glomerular EC the IGFBP4/IGFBP5 ratio controls the response of these cells to IGF-I and high levels of glucose, in terms of cellular growth.

Gene duplication and functional divergence of the zebrafish insulin-like growth factor 1 receptors

Insulin-like growth factor (IGF) 1 receptor (IGF1R)-mediated signaling plays key roles in growth, development, and physiology. Recent studies have shown that there are two distinct ig f1r genes in zebrafish, termed ig f1ra and ig f1rb. In this study, we tested the hypothesis that zebrafish ig f1ra and ig f1rb resulted from a gene duplication event at the ig f1r locus and that this has led to their functional divergence. The genomic structures of zebrafish ig f1ra and ig f1rb were determined and their loci mapped. While zebrafish ig f1ra has 21 exons and is located on linkage group (LG) 18, zebrafish ig f1rb has 22 exons and mapped to LG 7. There is a strong syntenic relationship between the two zebrafish genes and the human IG F1R gene. Using a MO-based loss-of-function approach, we show that both Igf1ra and Igf1rb are required for zebrafish embryo viability and proper growth and development. Although Igf1ra and Igf1rb demonstrated a large degree of functional overlap with regard to cell differentiation in the developing eye, inner ear, heart, and muscle, they also exhibited functional distinction involving a greater requirement for Igf1rb in spontaneous muscle contractility. These findings suggest that the duplicated zebrafish ig f1r genes play largely overlapping but not identical functional roles in early development and provide novel insight into the functional evolution of the IGF1R/insulin receptor gene family.

HNF factors form a network to regulate liver-enriched genes in zebrafish

Defects in some of liver-enriched genes in mammals will cause liver- and/or blood-related diseases. However, due to the fact that embryogenesis happens intrauterinally in the mammals, the function of these liver-enriched genes during liver organogenesis is poorly studied. We report here the identification of 129 genuine liver-enriched genes in adult zebrafish and show that, through in situ hybridization, 69 of these genes are also enriched in the embryonic liver. External embryogenesis coupled with the well-established morpholino-mediated gene knock-down technique in zebrafish offers us a unique opportunity to study if this group of genes plays any role during liver organogenesis in the future. As an example, preliminary study using morpholino-mediated gene knock-down method revealed that a novel liver-enriched gene leg1 is crucial for the liver expansion growth. We also report the analysis of promoter regions of 51 liver-enriched genes by searching putative binding sites for Hnf1, Hnf3, Hnf4 and Hnf6, four key transcription factors enriched in the liver. We found that promoter regions of majority of liver-enriched genes contain putative binding sites for more than one HNF factors, suggesting that most of liver-enriched genes are likely co-regulated by different combination of HNF factors. This observation supports the hypothesis that these four liver-enriched transcription factors form a network in controlling the expression of liver-specific or -enriched genes in the liver.

Prenatal exposure to excess testosterone modifies the developmental trajectory of the insulin-like growth factor system in female sheep

Experimental elevation of maternal testosterone (T) from 30 to 90 days of gestation leads to intrauterine growth retardation (IUGR) and increased prepubertal growth rate in female lambs. This study tested the hypothesis that prenatal T treatment during mid-gestation alters the trajectory of the fetal insulin-like growth factor (IGF)-insulin-like growth factor binding protein (IGFBP) system to promote IUGR and subsequent postnatal catch-up growth in female lambs. Plasma IGF-I and IGFBPs were measured by radioimmunoassay and Western ligand blot, respectively, on 65, 90 and 140 days (d) of gestation, at birth, approximately 5 months (prepubertal, the catch-up growth period), and approximately 9.5 months (postpubertal). Northern blot analysis was used to measure hepatic mRNA content of IGF system components during fetal stages. At fetal 65 d, plasma protein and hepatic mRNA content of IGFBP-1, an inhibitor of IGF bioactivity, was elevated in prenatal T-treated fetuses although body weight did not differ. There was a transient increase in plasma IGF-I and IGFBP-3 concentrations at fetal 90 d in prenatal T-treated fetuses. Hepatic IGF-I mRNA and plasma IGFBP-3 content were reduced by 140 d when body weight was reduced in prenatal T-treated fetuses. Plasma IGFBP-2 content was significantly reduced in prenatal T-treated newborns, but by 4 months these females had significantly higher circulating IGF-I and IGFBP-3 concentrations and faster growth rates than control females. After puberty, plasma IGF-I remained elevated in prenatal T-treated females. These findings provide evidence that prenatal T excess programmes the developmental trajectory of the IGF/IGFBP system in female sheep to reduce IGF bioavailability during IUGR and increase IGF bioavailability during prepubertal catch-up growth.

Roles of insulin-like growth factor (IGF) binding proteins in regulating IGF actions

The insulin-like growth factor (IGF) system is an evolutionarily conserved signaling pathway that is composed of two IGF ligands, two IGF receptors, and six IGF binding proteins. Studies in a variety of species suggest that the IGF signaling system plays a fundamental role in regulating embryonic growth and differentiation as well as in maintaining homeostasis in the adults. In extracellular fluids, IGFs are present in a complex with an IGF-binding protein (IGFBP). These IGFBPs are traditionally thought to function as carrier proteins and regulate circulating IGF turnover, transport, and distribution. Locally expressed IGFBPs can also inhibit and/or potentiate IGF activities. Recent studies have shown that some IGFBPs, in particular IGFBP-3 and -5, possess intrinsic biological activities and can act through IGF-independent mechanisms. In this article, we provide a brief overview of our current understanding of the IGF signaling system with particular reference to IGFBPs.